Define the intensity of a wave.

Intensity is the power per unit area — the energy passing through a unit area per unit time. The area is measured perpendicular to the wave's direction of travel.

Define a spherical wave .

A wave from a point source that spreads out equally in all directions .

As a spherical wave spreads out, its energy passes through an area equal to the .......... of a sphere, 4πr 2

As a spherical wave spreads out, its energy passes through an area equal to the surface area of a sphere, 4πr 2

True or False? The intensity of a wave decreases linearly with distance from the source.

False. Intensity follows an inverse square law ( I ∝ 1/ r 2 ), so it falls off far more rapidly than a linear decrease.

The change in direction of light as it passes the boundary between two transparent media The change in direction is caused by a change in speed .

What happens to light entering a more dense medium?

It slows down and bends towards the normal .

When light enters a less dense medium it speeds up and bends .......... the normal

When light enters a less dense medium it speeds up and bends away from the normal

True or False? When light travels along the normal into a new medium, its speed stays the same.

False. Light does not change direction along the normal, but its speed still changes because it enters a medium with a different refractive index.

Why is a material's refractive index always greater than 1?

The speed of light in a substance v is always less than its speed in a vacuum c , so n = c / v is always greater than 1 . For calculations, the refractive index of air is taken as 1 .

Define the critical angle , C .

The angle of incidence (at a more dense to less dense boundary) for which the angle of refraction is 90° At this angle the refracted ray travels along the boundary .

What is the angle of refraction when the angle of incidence equals the critical angle?

Exactly 90° — the light is refracted along the boundary .

A critical angle only exists when light travels from a more dense medium into a .......... one

A critical angle only exists when light travels from a more dense medium into a less dense one

True or False? The critical angle is measured as the angle of refraction .

False. The critical angle is a specific angle of incidence — the one for which the angle of refraction reaches 90°.

Define total internal reflection (TIR).

When all the light hitting a boundary is reflected back into the medium, with none refracted out It occurs when the angle of incidence exceeds the critical angle and the light is in the more dense medium.

For total internal reflection, the angle of incidence must be .......... the critical angle

For total internal reflection, the angle of incidence must be greater than the critical angle

Total internal reflection can only occur when light travels from a more dense medium into a .......... one, so that n 1 > n 2

Total internal reflection can only occur when light travels from a more dense medium into a less dense one, so that n 1 > n 2

True or False? During total internal reflection, only some of the light is reflected.

False. All of the light is reflected back into the more dense medium — none is refracted out.

What does the abbreviation TIR stand for?

Total Internal Reflection — give the full name in exam answers.

State the control variables for the refraction experiment.

The same perspex block The width of the light beam The same frequency / wavelength of the light

The angles of incidence i and refraction r are always measured from the ..........

The angles of incidence i and refraction r are always measured from the normal

In this experiment, how do the angles of incidence i and refraction r compare as light enters, and as it exits, the perspex block?

Entering the block: i > r (bends towards the normal) Exiting the block: i < r (bends away from the normal)

True or False? The angle of refraction is measured from the surface of the block.

False. Both the angle of incidence and the angle of refraction are measured from the normal — the line perpendicular to the surface.

Give one safety precaution for this ray box experiment.

Any one from: The ray box gets hot — avoid touching it to prevent burns Do not look directly into the light, as it may damage the eyes Keep liquids away from the electrical equipment

A converging lens that brings parallel rays of light to a focus by refraction

Define concave lens .

A diverging lens that makes parallel rays of light spread out from a point

Define focal length .

The distance from the lens to the principal focus

For a concave lens, the principal focus is the point from which the parallel rays appear to ..........

For a concave lens, the principal focus is the point from which the parallel rays appear to diverge

True or False? The more curved a lens, the longer its focal length

False. The more curved the lens, the shorter the focal length

An image formed by the convergence of rays of light; it can be projected onto a screen

Define virtual image .

An image that is seen but not formed on a screen ; the rays of light have not met but appear to come from a point

What three features are used to describe an image formed by a lens?

Whether it is: real or virtual bigger , the same size or smaller than the object inverted or the same way up as the object

Describe the image formed by a converging lens when the object is placed between one focal length (f) and two focal lengths (2f) from the lens.

The image is: real enlarged inverted

Describe the image formed by a converging lens when the object is placed further than two focal lengths (2f) from the lens.

The image is: real diminished (smaller) inverted

Describe the image formed by a converging lens when the object is placed at exactly two focal lengths (2f) from the lens.

The image is: real the same size as the object inverted

When an object is placed closer to a converging lens than its focal length, the image formed is virtual, enlarged and ..........

When an object is placed closer to a converging lens than its focal length, the image formed is virtual, enlarged and upright (as in a magnifying glass)

True or False? A virtual image can be projected onto a screen

False. Only a real image can be projected onto a screen, because it is formed where rays of light actually meet . A virtual image is only perceived by the eye

Define the power of a lens.

A measure of a lens's ability to refract light; the more refraction it causes, the higher its power

The power of a lens is .......... proportional to its focal length

The power of a lens is inversely proportional to its focal length

A diverging lens has a negative focal length. What is the sign of its power?

Negative — because power is the reciprocal of focal length, a negative focal length gives a negative power

True or False? The shorter the focal length, the lower the power

False. The shorter the focal length, the greater the power of the lens

Define compound lens .

Multiple lenses used in series (one after the other), whose powers add together

In a compound lens, the lenses should be arranged so that their .......... line up

In a compound lens, the lenses should be arranged so that their principal axes line up

How should the lenses in a compound lens be spaced?

They should be touching or very close together

True or False? The total power of a compound lens is found by multiplying the individual powers

False. The individual lens powers are added together, not multiplied

An image formed when light rays from a point on an object actually pass through another point in space The light rays are really there It can be formed on a screen

Define virtual image .

An image formed when light rays from a point on an object only appear to have come from another point in space The light rays are not really where the image appears to be It cannot be formed on a screen

How can a ray diagram show whether an image is real or virtual?

If the rays from the object naturally cross , the image is real If the rays must be extended backwards to make them cross, the image is virtual Virtual rays are drawn as dashed lines

A virtual image .......... be formed on a .......... .

A virtual image cannot be formed on a screen

A LevelPhysicsEdexcelFlashcards5. Waves & Particle Nature of LightRefraction, Reflection & Polarisation

Refraction, Reflection & Polarisation (Edexcel A Level Physics): Flashcards

Exam code: 9PH0

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FrontEquation for the Intensity of Radiation
Define the intensity of a wave.

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Cards in this collection (84)

Define the intensity of a wave.
Intensity is the power per unit area — the energy passing through a unit area per unit time.
The area is measured perpendicular to the wave's direction of travel.
Define a spherical wave.
A wave from a point source that spreads out equally in all directions.
How does the intensity of a progressive wave depend on its amplitude and frequency?
Intensity is proportional to amplitude squared and frequency squared
$I \propto A^{2} f^{2}$
So doubling either the amplitude or the frequency increases intensity by a factor of 4.
As a spherical wave spreads out, its energy passes through an area equal to the .......... of a sphere, 4πr²
As a spherical wave spreads out, its energy passes through an area equal to the surface area of a sphere, 4πr²
How does the intensity from a point source vary with distance r?
It obeys the inverse square law — intensity is proportional to 1/r²
$I \propto \frac{1}{r^{2}}$
When the source is twice as far away, the intensity is four times smaller.
True or False?
The intensity of a wave decreases linearly with distance from the source.
False.
Intensity follows an inverse square law (I ∝ 1/r²), so it falls off far more rapidly than a linear decrease.
What is meant by the inverse square law for intensity?
The intensity is inversely proportional to the square of the distance from the source
$I \propto \frac{1}{r^{2}}$
This assumes there is no absorption of the wave's energy.
Define refraction.
The change in direction of light as it passes the boundary between two transparent media
The change in direction is caused by a change in speed.
Define refractive index, n.
A measure of how much light slows down when passing through a material
$n = \frac{c}{v}$
where c is the speed of light in a vacuum and v is the speed of light in the substance.
What happens to light entering a more dense medium?
It slows down and bends towards the normal.
When light enters a less dense medium it speeds up and bends .......... the normal
When light enters a less dense medium it speeds up and bends away from the normal
State Snell's law.
$n_{1} \sin θ_{1} = n_{2} \sin θ_{2}$
Angles are measured from the normal; material 1 is the medium the ray travels through first.
True or False?
When light travels along the normal into a new medium, its speed stays the same.
False.
Light does not change direction along the normal, but its speed still changes because it enters a medium with a different refractive index.
Why is a material's refractive index always greater than 1?
The speed of light in a substance v is always less than its speed in a vacuum c, so n = c/v is always greater than 1.
For calculations, the refractive index of air is taken as 1.
Define the critical angle, C.
The angle of incidence (at a more dense to less dense boundary) for which the angle of refraction is 90°
At this angle the refracted ray travels along the boundary.
What is the angle of refraction when the angle of incidence equals the critical angle?
Exactly 90° — the light is refracted along the boundary.
Give the formula for the critical angle C derived from Snell's law.
$\sin C = \frac{n_{2}}{n_{1}}$
Found from n₁ sin θ₁ = n₂ sin θ₂ with θ₁ = C and θ₂ = 90°
For a material–air boundary this becomes sin C = 1/n.
A critical angle only exists when light travels from a more dense medium into a .......... one
A critical angle only exists when light travels from a more dense medium into a less dense one
True or False?
The critical angle is measured as the angle of refraction.
False.
The critical angle is a specific angle of incidence — the one for which the angle of refraction reaches 90°.
A glass cube (n = 1.45) is in contact with a liquid (n = 1.32). Calculate the critical angle at the glass–liquid boundary.
$\sin C = \frac{n_{2}}{n_{1}} = \frac{1.32}{1.45}$
$C = \sin^{- 1} (\frac{1.32}{1.45}) = 65.6 °$
Define total internal reflection (TIR).
When all the light hitting a boundary is reflected back into the medium, with none refracted out
It occurs when the angle of incidence exceeds the critical angle and the light is in the more dense medium.
For total internal reflection, the angle of incidence must be .......... the critical angle
For total internal reflection, the angle of incidence must be greater than the critical angle
Total internal reflection can only occur when light travels from a more dense medium into a .......... one, so that n₁ > n₂
Total internal reflection can only occur when light travels from a more dense medium into a less dense one, so that n₁ > n₂
True or False?
During total internal reflection, only some of the light is reflected.
False.
All of the light is reflected back into the more dense medium — none is refracted out.
What does the abbreviation TIR stand for?
Total Internal Reflection — give the full name in exam answers.
What is the dependent variable when measuring the refractive index of perspex?
The angle of refraction, r.
State the control variables for the refraction experiment.
- The same perspex block
- The width of the light beam
- The same frequency / wavelength of the light
The angles of incidence i and refraction r are always measured from the ..........
The angles of incidence i and refraction r are always measured from the normal
In this experiment, how do the angles of incidence i and refraction r compare as light enters, and as it exits, the perspex block?
- Entering the block: i > r (bends towards the normal)
- Exiting the block: i < r (bends away from the normal)
True or False?
The angle of refraction is measured from the surface of the block.
False.
Both the angle of incidence and the angle of refraction are measured from the normal — the line perpendicular to the surface.
Give one safety precaution for this ray box experiment.
Any one from:
- The ray box gets hot — avoid touching it to prevent burns
- Do not look directly into the light, as it may damage the eyes
- Keep liquids away from the electrical equipment
Define convex lens.
A converging lens that brings parallel rays of light to a focus by refraction
Define concave lens.
A diverging lens that makes parallel rays of light spread out from a point
Define focal length.
The distance from the lens to the principal focus
By what process does a lens form an image?
By refracting light
For a concave lens, the principal focus is the point from which the parallel rays appear to ..........
For a concave lens, the principal focus is the point from which the parallel rays appear to diverge
True or False?
The more curved a lens, the longer its focal length
False.
The more curved the lens, the shorter the focal length
Define real image.
An image formed by the convergence of rays of light; it can be projected onto a screen
Define virtual image.
An image that is seen but not formed on a screen; the rays of light have not met but appear to come from a point
What three features are used to describe an image formed by a lens?
Whether it is:
- real or virtual
- bigger, the same size or smaller than the object
- inverted or the same way up as the object
Describe the image formed by a converging lens when the object is placed between one focal length (f) and two focal lengths (2f) from the lens.
The image is:
- real
- enlarged
- inverted
Describe the image formed by a converging lens when the object is placed further than two focal lengths (2f) from the lens.
The image is:
- real
- diminished (smaller)
- inverted
Describe the image formed by a converging lens when the object is placed at exactly two focal lengths (2f) from the lens.
The image is:
- real
- the same size as the object
- inverted
When an object is placed closer to a converging lens than its focal length, the image formed is virtual, enlarged and ..........
When an object is placed closer to a converging lens than its focal length, the image formed is virtual, enlarged and upright (as in a magnifying glass)
True or False?
A virtual image can be projected onto a screen
False.
Only a real image can be projected onto a screen, because it is formed where rays of light actually meet. A virtual image is only perceived by the eye
Define the power of a lens.
A measure of a lens's ability to refract light; the more refraction it causes, the higher its power
State the equation for the power of a lens.
$P = \frac{1}{f}$
- $P$ = power (dioptres, D)
- $f$ = focal length (m)
State the unit of lens power.
The dioptre (D)
The power of a lens is .......... proportional to its focal length
The power of a lens is inversely proportional to its focal length
A diverging lens has a negative focal length. What is the sign of its power?
Negative — because power is the reciprocal of focal length, a negative focal length gives a negative power
True or False?
The shorter the focal length, the lower the power
False.
The shorter the focal length, the greater the power of the lens
Define compound lens.
Multiple lenses used in series (one after the other), whose powers add together
How do you find the total power of a compound lens?
Find the sum of the powers of the individual lenses:
$P_{Total} = P_{1} + P_{2} + \dots + P_{n}$
In a compound lens, the lenses should be arranged so that their .......... line up
In a compound lens, the lenses should be arranged so that their principal axes line up
How should the lenses in a compound lens be spaced?
They should be touching or very close together
True or False?
The total power of a compound lens is found by multiplying the individual powers
False.
The individual lens powers are added together, not multiplied
Define real image.
An image formed when light rays from a point on an object actually pass through another point in space
- The light rays are really there
- It can be formed on a screen
Define virtual image.
An image formed when light rays from a point on an object only appear to have come from another point in space
- The light rays are not really where the image appears to be
- It cannot be formed on a screen
How can a ray diagram show whether an image is real or virtual?
- If the rays from the object naturally cross, the image is real
- If the rays must be extended backwards to make them cross, the image is virtual
- Virtual rays are drawn as dashed lines
A virtual image .......... be formed on a ...........
A virtual image cannot be formed on a screen
True or False?
A diverging lens can form a real image.
False.
The image formed by a diverging (concave) lens is always virtual, upright and diminished.
Describe the image formed by a converging lens when the object is placed between f and 2f.
The image is:
- Real
- Enlarged
- Inverted
Describe the image formed by a converging lens used as a magnifying glass (object closer than f).
The image is:
- Virtual
- Enlarged
- Upright
Describe the image formed by a diverging (concave) lens.
The image is:
- Virtual
- Diminished
- Upright
State the thin lens equation.
$\frac{1}{f} = \frac{1}{u} + \frac{1}{v}$
In the lens equation, what do f, u and v represent?
- f = focal length of the lens
- u = object distance from the lens
- v = image distance from the lens
The lens equation can only be applied to .......... converging and diverging lenses.
The lens equation can only be applied to thin converging and diverging lenses
In the lens equation, when is the image distance v taken as positive?
When the image is real
After using the lens equation to find $\frac{1}{f}$ , what final step is needed to get f?
Take the reciprocal
- The equation gives $\frac{1}{f}$ , so you must invert your result to find the focal length f
True or False?
The lens equation works only for converging lenses.
False.
It applies to all thin converging and diverging lenses.
Define magnification.
How much larger the image is than the object
- It is the ratio of the image height to the object height
Give the two formulae used to calculate magnification m.
$m = \frac{h_{i}}{h_{o}}$ (image height ÷ object height)
$m = \frac{v}{u}$ (image distance ÷ object distance)
What does a positive value of magnification tell you about the image?
The image is real and inverted
What does a negative value of magnification tell you about the image?
The image is virtual and upright
Because magnification is a ratio, it has ...........
Because magnification is a ratio, it has no units
True or False?
Distances must be in metres to find magnification.
False.
Because magnification is a ratio, the units cancel, so distances need only be in the same unit (e.g. all in cm) — they do not have to be converted to SI units.
A magnification greater than 1 means the image is .......... than the object.
A magnification greater than 1 means the image is larger than the object
Define polarisation.
When particles are only allowed to oscillate in one of the directions perpendicular to the direction of wave propagation
Define an unpolarised wave.
A transverse wave whose oscillations occur in any plane perpendicular to the direction of motion and energy transfer of the wave
True or False?
Longitudinal waves can be polarised.
False.
Longitudinal waves oscillate in the same direction as the wave's motion, so polarisation cannot occur — only transverse waves can be polarised.
For an electromagnetic wave, which oscillation defines its plane of polarisation?
The plane of the electric field oscillation
Light waves can be polarised by passing them through a ...........
Light waves can be polarised by passing them through a polarising filter (polariser)
Name two methods of polarising light.
- Using a polarising filter
- Reflection from a non-metallic plane surface
What happens when vertically polarised light meets a filter with a horizontal transmission axis?
No light is transmitted
- The wave is blocked completely

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