Define intensity of a progressive wave.

Intensity is the power transmitted per unit area , where the area is perpendicular to the direction of the wave's velocity.

If the amplitude of a wave is doubled, by what factor does its intensity change?

The intensity increases by a factor of 4 (2 2 ), since intensity is proportional to amplitude squared.

For a spherical wave, intensity I is proportional to , which is known as the inverse square law.

For a spherical wave, intensity I is proportional to 1/r 2 , which is known as the inverse square law.

True or False? If a point source is moved to twice its original distance away, the intensity received halves.

False. Intensity follows an inverse square law , so doubling the distance reduces the intensity by a factor of 4 , not 2.

Refraction is the change in direction and speed of light as it passes across a boundary between two different transparent media.

A light ray travelling through air enters a glass block. Describe what happens to its speed and direction.

The light ray slows down , since glass is more optically dense than air, and it bends towards the normal .

A ray of light travelling along the normal to a boundary changes but does not change direction.

A ray of light travelling along the normal to a boundary changes speed but does not change direction.

True or False? A calculated refractive index of 0.8 could be correct for a glass sample.

False. Since the speed of light in a substance is always less than in a vacuum, refractive index must always be greater than 1 . A value below 1 signals a calculation error.

Define the critical angle , C .

The critical angle is the angle of incidence, in the more dense medium, at which the angle of refraction is exactly 90° , so the refracted ray travels along the boundary.

Can the critical angle be observed for light travelling from air into glass? Explain your answer.

No. The critical angle can only occur when light travels from a more dense to a less dense medium, and air-to-glass is the opposite direction.

At the critical angle, the angle of refraction is exactly .

At the critical angle, the angle of refraction is exactly 90° .

Define total internal reflection (TIR).

Total internal reflection occurs when the angle of incidence is greater than the critical angle, and the incident refractive index n 1 is greater than the refractive index of the material at the boundary n 2 , so all of the light is reflected back into the first medium.

State the two conditions needed for total internal reflection to occur.

The light must be travelling from a more dense medium into a less dense medium The angle of incidence must be greater than the critical angle

A ray of light in glass strikes a glass-air boundary at an angle of incidence smaller than the critical angle. Does total internal reflection occur? Explain.

No. Since the angle of incidence is less than the critical angle, the light refracts and partially transmits into the air, rather than being totally internally reflected.

Total internal reflection can only occur when light travels from a medium into a less dense one.

Total internal reflection can only occur when light travels from a more dense medium into a less dense one.

True or False? "Internal reflection" is an acceptable way to name the phenomenon in an exam answer.

False. The full name, Total Internal Reflection , must be given — an incomplete name may not gain credit.

State the aim of the experiment to measure the refractive index of a perspex block.

To investigate the refraction of light through a perspex block by measuring the angle of refraction, r , for different angles of incidence, i .

In the refractive index experiment, why must the perspex block, light beam width and light frequency all be kept the same across repeats?

These are the control variables — keeping them constant ensures that only the angle of incidence affects the angle of refraction, so results can be compared fairly.

Describe how the entry and exit points of the light ray are marked on the paper during the refractive index experiment.

Small 'x' marks are made at: a point on the incident ray near the ray box, where the ray enters the block, where the ray exits the block, and a point about 5 cm along the exit ray.

For a light ray exiting the perspex block, the angle of refraction is than the angle of incidence.

For a light ray exiting the perspex block, the angle of refraction is greater than the angle of incidence.

True or False? When the angle of incidence at the perspex surface is 90° to the block, the ray refracts sharply.

False. At 90° to the surface (along the normal), the ray passes straight through without changing direction , so i = r = 0°.

State one safety precaution needed when using a ray box in this experiment.

Any one from: Avoid looking directly at the light, as it may damage the eyes Stand behind the ray box during the experiment Keep liquids away from the electrical equipment, as the ray box gets hot

Define plane polarisation .

A wave is plane polarised when its particle oscillations occur in a single plane perpendicular to the direction of wave propagation.

Why can only transverse waves be polarised, not longitudinal waves?

Transverse waves oscillate in any plane perpendicular to the direction of travel, so they can be restricted to one plane. Longitudinal waves oscillate parallel to the direction of travel, so there is no perpendicular plane to restrict.

State three methods by which light can become polarised.

Polarising filters Reflection (from a non-metallic surface) Refraction

Explain why Polaroid sunglasses with vertically aligned lenses reduce glare from a wet road.

Light reflected from a horizontal surface is partially polarised in the plane parallel to the surface, i.e. horizontally . Vertically aligned Polaroid lenses block this horizontally polarised light, reducing glare.

Light reflected from a non-metallic surface is polarised in a plane to the reflecting surface.

Light reflected from a non-metallic surface is polarised in a plane parallel to the reflecting surface.

True or False? Refracted light is polarised in a plane parallel to the transmitting surface.

False. Refracted light is polarised in a plane perpendicular to the transmitting surface, not parallel.

How can polarising filters be used to detect stress concentrations in a transparent material?

The sample is placed between two polarising filters at 90° to each other. Stressed regions rotate the plane of polarisation, allowing light through; brighter areas indicate more stress.

International A Level (IAL)PhysicsEdexcelFlashcards2. Waves & ElectricityRefraction, Reflection & Polarisation

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

Exam code: YPH11

1/35

0Still learning

Know0

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

Was this flashcard helpful?

Cards in this collection (35)

Define intensity of a progressive wave.
Intensity is the power transmitted per unit area, where the area is perpendicular to the direction of the wave's velocity.
How is intensity related to amplitude and frequency?
Intensity is proportional to amplitude squared and frequency squared:
$I \propto A^{2} f^{2}$
If the amplitude of a wave is doubled, by what factor does its intensity change?
The intensity increases by a factor of 4 (2²), since intensity is proportional to amplitude squared.
A point source emits a spherical wave. Explain, in terms of area, why the intensity of the wave decreases with distance from the source.
As the wave travels outward, its energy spreads over the surface area of a sphere, $4 π r^{2}$ . Since this area increases with distance, the same power is spread over a larger area, so intensity decreases.
For a spherical wave, intensity I is proportional to , which is known as the inverse square law.
For a spherical wave, intensity I is proportional to 1/r², which is known as the inverse square law.
True or False?
If a point source is moved to twice its original distance away, the intensity received halves.
False.
Intensity follows an inverse square law, so doubling the distance reduces the intensity by a factor of 4, not 2.
Define refraction.
Refraction is the change in direction and speed of light as it passes across a boundary between two different transparent media.
Define refractive index, n.
The refractive index of a material measures how much light slows down when passing through it:
$n = \frac{c}{v}$
where c is the speed of light in a vacuum and v is the speed of light in the substance.
State Snell's law.
$n_{1} \sin θ_{1} = n_{2} \sin θ_{2}$
where n₁ and n₂ are the refractive indices of the first and second material, and θ₁ and θ₂ are the angles of incidence and refraction, both measured from the normal.
A light ray travelling through air enters a glass block. Describe what happens to its speed and direction.
The light ray slows down, since glass is more optically dense than air, and it bends towards the normal.
A ray of light travelling along the normal to a boundary changes but does not change direction.
A ray of light travelling along the normal to a boundary changes speed but does not change direction.
True or False?
A calculated refractive index of 0.8 could be correct for a glass sample.
False.
Since the speed of light in a substance is always less than in a vacuum, refractive index must always be greater than 1. A value below 1 signals a calculation error.
Define the critical angle, C.
The critical angle is the angle of incidence, in the more dense medium, at which the angle of refraction is exactly 90°, so the refracted ray travels along the boundary.
Derive an expression for the critical angle, C, at a boundary between a medium of refractive index n and air.
From Snell's law with θ₁ = C, θ₂ = 90°, n₁ = n, n₂ = 1:
$n \sin C = \sin 90 °$
$C = \sin^{- 1} (\frac{1}{n})$
Can the critical angle be observed for light travelling from air into glass? Explain your answer.
No. The critical angle can only occur when light travels from a more dense to a less dense medium, and air-to-glass is the opposite direction.
At the critical angle, the angle of refraction is exactly .
At the critical angle, the angle of refraction is exactly 90°.
True or False?
A material with a higher refractive index has a larger critical angle.
False.
Since $C = \sin^{- 1} (1 / n)$ , a higher refractive index gives a smaller critical angle.
Define total internal reflection (TIR).
Total internal reflection occurs when the angle of incidence is greater than the critical angle, and the incident refractive index n₁ is greater than the refractive index of the material at the boundary n₂, so all of the light is reflected back into the first medium.
State the two conditions needed for total internal reflection to occur.
- The light must be travelling from a more dense medium into a less dense medium
- The angle of incidence must be greater than the critical angle
A ray of light in glass strikes a glass-air boundary at an angle of incidence smaller than the critical angle. Does total internal reflection occur? Explain.
No. Since the angle of incidence is less than the critical angle, the light refracts and partially transmits into the air, rather than being totally internally reflected.
Total internal reflection can only occur when light travels from a medium into a less dense one.
Total internal reflection can only occur when light travels from a more dense medium into a less dense one.
True or False?
"Internal reflection" is an acceptable way to name the phenomenon in an exam answer.
False.
The full name, Total Internal Reflection, must be given — an incomplete name may not gain credit.
State the aim of the experiment to measure the refractive index of a perspex block.
To investigate the refraction of light through a perspex block by measuring the angle of refraction, r, for different angles of incidence, i.
In the refractive index experiment, why must the perspex block, light beam width and light frequency all be kept the same across repeats?
These are the control variables — keeping them constant ensures that only the angle of incidence affects the angle of refraction, so results can be compared fairly.
Describe how the entry and exit points of the light ray are marked on the paper during the refractive index experiment.
Small 'x' marks are made at: a point on the incident ray near the ray box, where the ray enters the block, where the ray exits the block, and a point about 5 cm along the exit ray.
For a light ray exiting the perspex block, the angle of refraction is than the angle of incidence.
For a light ray exiting the perspex block, the angle of refraction is greater than the angle of incidence.
True or False?
When the angle of incidence at the perspex surface is 90° to the block, the ray refracts sharply.
False.
At 90° to the surface (along the normal), the ray passes straight through without changing direction, so i = r = 0°.
State one safety precaution needed when using a ray box in this experiment.
Any one from:
- Avoid looking directly at the light, as it may damage the eyes
- Stand behind the ray box during the experiment
- Keep liquids away from the electrical equipment, as the ray box gets hot
Define plane polarisation.
A wave is plane polarised when its particle oscillations occur in a single plane perpendicular to the direction of wave propagation.
Why can only transverse waves be polarised, not longitudinal waves?
Transverse waves oscillate in any plane perpendicular to the direction of travel, so they can be restricted to one plane. Longitudinal waves oscillate parallel to the direction of travel, so there is no perpendicular plane to restrict.
State three methods by which light can become polarised.
- Polarising filters
- Reflection (from a non-metallic surface)
- Refraction
Explain why Polaroid sunglasses with vertically aligned lenses reduce glare from a wet road.
Light reflected from a horizontal surface is partially polarised in the plane parallel to the surface, i.e. horizontally. Vertically aligned Polaroid lenses block this horizontally polarised light, reducing glare.
Light reflected from a non-metallic surface is polarised in a plane to the reflecting surface.
Light reflected from a non-metallic surface is polarised in a plane parallel to the reflecting surface.
True or False?
Refracted light is polarised in a plane parallel to the transmitting surface.
False.
Refracted light is polarised in a plane perpendicular to the transmitting surface, not parallel.
How can polarising filters be used to detect stress concentrations in a transparent material?
The sample is placed between two polarising filters at 90° to each other. Stressed regions rotate the plane of polarisation, allowing light through; brighter areas indicate more stress.

Sign up to unlock flashcards

By signing up you agree to our Terms and Privacy Policy

Previous:Interference & Stationary WavesNext:Waves, Electrons & Photons