Define converging lens .

A lens that refracts parallel rays of light so they are brought to a focus at a single point on the principal axis, called the focal point .

Define focal length .

The distance from the centre of a lens to the focal point .

What effect does increasing the curvature (thickness) of a lens have on its focal length and power?

A more curved , thicker lens has a shorter focal length and is more powerful .

True or False? A diverging lens has a positive focal length.

False. The focal length of a diverging lens is negative , because its focal point is behind the lens (on the same side as the object). A converging lens has a positive focal length.

State the three properties used to describe an image formed by a lens.

Nature : real or virtual Orientation : inverted or upright (compared to the object) Size : magnified, diminished or the same size (compared to the object)

State three differences between a real image and a virtual image.

A real image can be projected onto a screen; a virtual image cannot A real image is always inverted ; a virtual image is always upright A real image forms at the intersection of two solid lines; a virtual image forms at the intersection of a dashed line (or two dashed lines) with a solid line

Rays travelling parallel to the principal axis are refracted by the lens and pass through the .......... .

Rays travelling parallel to the principal axis are refracted by the lens and pass through the focal point .

For a converging lens, what is the nature, orientation and size of the image when the object is placed closer to the lens than the focal length (as in a magnifying glass)?

The image is virtual , upright and magnified .

State the nature, orientation, size and position of the image formed by a diverging lens, whatever the position of the object.

Nature : virtual Orientation : upright Size : diminished Position : on the same side of the lens as the object

The power of a lens is .......... proportional to its focal length; the shorter the focal length, the more powerful the lens.

The power of a lens is inversely proportional to its focal length; the shorter the focal length, the more powerful the lens.

What is the sign of the power of a diverging lens, and why?

Negative , because the focal length of a diverging (concave) lens is negative.

State the sign convention used in the lens equation for real images, virtual images and diverging lenses.

Values are positive if the image is real v is negative if the image is virtual f is negative if the lens is diverging

In the lens equation, u is the distance from the object to the lens and v is the distance from the .......... to the lens.

In the lens equation, u is the distance from the object to the lens and v is the distance from the image to the lens.

Must f , u and v all be measured in metres when using the lens equation?

No. They do not all have to be in metres, but they must be in the same units as each other.

A transparent, convex membrane covering the front of the eye. It has a high refractive index , so it causes the largest amount of refraction of light entering the eye.

What happens to the iris and pupil in a dark room, and why?

The iris expands , causing the pupil to dilate (widen), so that more light can enter the eye.

The closest distance the eye can focus on. For a 'normal-sighted' person this is 25 cm .

The furthest distance the eye can comfortably focus on. For a 'normal-sighted' person this is infinity .

What happens to the lens when the ciliary muscles contract, and when does this occur?

The lens becomes wider , more spherical and more powerful . This occurs when focusing on an object close to the near point .

What happens to the lens when the ciliary muscles relax, and what is this state called?

The lens becomes thinner , less spherical and less powerful — called an unaccommodated lens. This occurs when focusing on an object close to the far point .

How is the total power of the eye calculated?

By adding together the powers of the cornea and the lens .

True or False? The image formed on the retina is the right way up.

False. The image on the retina is inverted (upside down). The brain interprets it as being the correct way up.

Approximately how many times more rods than cones are there in the human retina?

About twenty times more rods than cones.

Define photoreceptors .

Light-sensitive cells in the retina ( rods and cones ) that detect light and convert it into an electrical signal sent to the brain.

What causes a rod or cone to send a signal to the brain?

Light falling on the chemical pigments in the cell bleaches them, which stimulates the cell to send a signal via the optic nerve .

State three differences between rods and cones.

Rods work well in low-intensity light; cones need high-intensity light Rods contribute little detail ; cones contribute greater detail Rods are not sensitive to colour ; cones detect colour

There is one type of .......... but three types of .......... , each sensitive to a different wavelength range: red, green and blue.

There is one type of rod but three types of cone , each sensitive to a different wavelength range: red, green and blue.

True or False? The eye is equally responsive to blue, red and green light.

False. The eye is less responsive to blue light than to red or green, so blue light appears dimmer .

How is yellow light detected by the eye?

By combining signals from the red and green cones.

What determines the colour interpreted by the brain?

The brain processes the weighted relative strengths of the signals from the three types of cone (red, green, blue) and interprets this as colour.

A measure of the amount of detail that can be seen in an image. The greater the resolution, the greater the detail.

Define spatial resolution (of the eye).

The eye's ability to form separate images of objects that are close together.

Where are small details in an image best seen in high-intensity light?

By the rods and cones on the fovea , in the centre of vision (rather than the periphery).

Where are small details in an image best seen in low-intensity light?

By the rods and cones in the periphery (outer regions) of vision.

State the two conditions that must be met for the brain to resolve two separate light sources on the retina.

There must be at least one unbleached rod and one unbleached cone between the two stimulated cells The cell between them must not share an optic nerve with either of the two stimulated cells

True or False? Two objects will always be seen as separate as long as their light falls on different cells on the retina.

False. The objects will still be seen as one unless there is at least one unbleached rod and cone between the two stimulated cells, and that cell does not share an optic nerve with either of them.

If two light sources stimulate cells on the retina that .......... the same optic nerve, the brain cannot resolve them as separate objects.

If two light sources stimulate cells on the retina that share the same optic nerve, the brain cannot resolve them as separate objects.

Define myopia (short-sightedness).

A defect of vision where the lens is too curved (cornea and lens too powerful) or the eyeball is too long, so light is refracted too much and focuses in front of the retina . Distant objects appear blurry.

Define hyperopia (long-sightedness).

A defect of vision where the lens is less curved (cornea and lens too weak) or the eyeball is too short, so light is not refracted enough and focuses behind the retina . Close objects appear blurry.

What type of lens corrects short-sightedness, and how does it work?

A diverging (concave) lens. It causes light rays to diverge before reaching the eye, so the image forms on the retina instead of in front of it.

What type of lens corrects long-sightedness, and how does it work?

A converging (convex) lens. It causes light rays to converge before reaching the eye, so the image forms on the retina instead of behind it.

To correct short-sightedness, the correcting lens must have its .......... in the same position as the eye's faulty far point.

To correct short-sightedness, the correcting lens must have its principal focus in the same position as the eye's faulty far point.

True or False? A short-sighted person needs a converging lens to correct their vision.

False. Short-sightedness is corrected with a diverging lens, which causes light rays to diverge before entering the eye so the image forms on the retina instead of in front of it.

A defect of vision caused by an irregularly shaped (non-spherical) cornea , which gives different focal lengths for different planes of vision.

What is meant by the cylindrical power (CYL) of an astigmatism?

The difference in power between the two points of focus of the two planes of light. The greater the difference, the greater the astigmatism. Measured in dioptres (D) .

What three pieces of information does a prescription for astigmatism give for each eye?

Sphere (SPH) — the focal power needed to correct long- or short-sightedness Cylinder (CYL) — the cylindrical power needed to correct the astigmatism Axis angle — the angle needed by the lens to correct the astigmatism

How does the axis angle of a prescription relate to the angle of astigmatism on the cornea?

The axis angle of the correcting lens is perpendicular (90°) to the angle of astigmatism on the cornea.

A LevelPhysicsAQAFlashcardsMedical PhysicsPhysics of the Eye

Physics of the Eye (AQA A Level Physics): Flashcards

Exam code: 7408

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FrontConverging & Diverging Lenses
Define converging lens.

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Cards in this collection (54)

Define converging lens.
A lens that refracts parallel rays of light so they are brought to a focus at a single point on the principal axis, called the focal point.
Define focal length.
The distance from the centre of a lens to the focal point.
What effect does increasing the curvature (thickness) of a lens have on its focal length and power?
A more curved, thicker lens has a shorter focal length and is more powerful.
True or False?
A diverging lens has a positive focal length.
False.
The focal length of a diverging lens is negative, because its focal point is behind the lens (on the same side as the object). A converging lens has a positive focal length.
State the three properties used to describe an image formed by a lens.
- Nature: real or virtual
- Orientation: inverted or upright (compared to the object)
- Size: magnified, diminished or the same size (compared to the object)
State three differences between a real image and a virtual image.
- A real image can be projected onto a screen; a virtual image cannot
- A real image is always inverted; a virtual image is always upright
- A real image forms at the intersection of two solid lines; a virtual image forms at the intersection of a dashed line (or two dashed lines) with a solid line
Rays travelling parallel to the principal axis are refracted by the lens and pass through the ...........
Rays travelling parallel to the principal axis are refracted by the lens and pass through the focal point.
For a converging lens, what is the nature, orientation and size of the image when the object is placed closer to the lens than the focal length (as in a magnifying glass)?
The image is virtual, upright and magnified.
State the nature, orientation, size and position of the image formed by a diverging lens, whatever the position of the object.
- Nature: virtual
- Orientation: upright
- Size: diminished
- Position: on the same side of the lens as the object
Define power (of a lens).
A measure of how strongly a lens converges or diverges light, given by $P = \frac{1}{f}$ , where f is the focal length in metres. Power is measured in dioptres (D).
The power of a lens is .......... proportional to its focal length; the shorter the focal length, the more powerful the lens.
The power of a lens is inversely proportional to its focal length; the shorter the focal length, the more powerful the lens.
What is the sign of the power of a diverging lens, and why?
Negative, because the focal length of a diverging (concave) lens is negative.
State the sign convention used in the lens equation for real images, virtual images and diverging lenses.
- Values are positive if the image is real
- v is negative if the image is virtual
- f is negative if the lens is diverging
In the lens equation, u is the distance from the object to the lens and v is the distance from the .......... to the lens.
In the lens equation, u is the distance from the object to the lens and v is the distance from the image to the lens.
How is linear magnification defined in terms of heights?
$m = \frac{h_{i}}{h_{o}}$
The ratio of image height to object height.
How can magnification also be calculated using distances, and does it have units?
$m = \frac{v}{u}$
The ratio of image distance to object distance. Since magnification is a ratio, it has no units.
Must f, u and v all be measured in metres when using the lens equation?
No. They do not all have to be in metres, but they must be in the same units as each other.
Define cornea.
A transparent, convex membrane covering the front of the eye. It has a high refractive index, so it causes the largest amount of refraction of light entering the eye.
What happens to the iris and pupil in a dark room, and why?
The iris expands, causing the pupil to dilate (widen), so that more light can enter the eye.
Define near point.
The closest distance the eye can focus on. For a 'normal-sighted' person this is 25 cm.
Define far point.
The furthest distance the eye can comfortably focus on. For a 'normal-sighted' person this is infinity.
What happens to the lens when the ciliary muscles contract, and when does this occur?
The lens becomes wider, more spherical and more powerful. This occurs when focusing on an object close to the near point.
What happens to the lens when the ciliary muscles relax, and what is this state called?
The lens becomes thinner, less spherical and less powerful — called an unaccommodated lens. This occurs when focusing on an object close to the far point.
How is the total power of the eye calculated?
By adding together the powers of the cornea and the lens.
True or False?
The image formed on the retina is the right way up.
False.
The image on the retina is inverted (upside down). The brain interprets it as being the correct way up.
Approximately how many times more rods than cones are there in the human retina?
About twenty times more rods than cones.
Define photoreceptors.
Light-sensitive cells in the retina (rods and cones) that detect light and convert it into an electrical signal sent to the brain.
What causes a rod or cone to send a signal to the brain?
Light falling on the chemical pigments in the cell bleaches them, which stimulates the cell to send a signal via the optic nerve.
How are rods and cones reset (unbleached)?
By vitamin A from the blood.
State three differences between rods and cones.
- Rods work well in low-intensity light; cones need high-intensity light
- Rods contribute little detail; cones contribute greater detail
- Rods are not sensitive to colour; cones detect colour
How many nerve fibres is each cone connected to the brain by?
One nerve fibre per cone.
There is one type of .......... but three types of .........., each sensitive to a different wavelength range: red, green and blue.
There is one type of rod but three types of cone, each sensitive to a different wavelength range: red, green and blue.
True or False?
The eye is equally responsive to blue, red and green light.
False.
The eye is less responsive to blue light than to red or green, so blue light appears dimmer.
How is yellow light detected by the eye?
By combining signals from the red and green cones.
What determines the colour interpreted by the brain?
The brain processes the weighted relative strengths of the signals from the three types of cone (red, green, blue) and interprets this as colour.
Define resolution.
A measure of the amount of detail that can be seen in an image. The greater the resolution, the greater the detail.
Define spatial resolution (of the eye).
The eye's ability to form separate images of objects that are close together.
Where are small details in an image best seen in high-intensity light?
By the rods and cones on the fovea, in the centre of vision (rather than the periphery).
Where are small details in an image best seen in low-intensity light?
By the rods and cones in the periphery (outer regions) of vision.
State the two conditions that must be met for the brain to resolve two separate light sources on the retina.
- There must be at least one unbleached rod and one unbleached cone between the two stimulated cells
- The cell between them must not share an optic nerve with either of the two stimulated cells
True or False?
Two objects will always be seen as separate as long as their light falls on different cells on the retina.
False.
The objects will still be seen as one unless there is at least one unbleached rod and cone between the two stimulated cells, and that cell does not share an optic nerve with either of them.
If two light sources stimulate cells on the retina that .......... the same optic nerve, the brain cannot resolve them as separate objects.
If two light sources stimulate cells on the retina that share the same optic nerve, the brain cannot resolve them as separate objects.
Define myopia (short-sightedness).
A defect of vision where the lens is too curved (cornea and lens too powerful) or the eyeball is too long, so light is refracted too much and focuses in front of the retina. Distant objects appear blurry.
Define hyperopia (long-sightedness).
A defect of vision where the lens is less curved (cornea and lens too weak) or the eyeball is too short, so light is not refracted enough and focuses behind the retina. Close objects appear blurry.
What type of lens corrects short-sightedness, and how does it work?
A diverging (concave) lens. It causes light rays to diverge before reaching the eye, so the image forms on the retina instead of in front of it.
What type of lens corrects long-sightedness, and how does it work?
A converging (convex) lens. It causes light rays to converge before reaching the eye, so the image forms on the retina instead of behind it.
To correct short-sightedness, the correcting lens must have its .......... in the same position as the eye's faulty far point.
To correct short-sightedness, the correcting lens must have its principal focus in the same position as the eye's faulty far point.
A short-sighted person has a far point of 8 m. Calculate the power of the lens needed to correct their vision.
f = far point = −8 m (focal length of a diverging lens is negative)
$P = \frac{1}{f} = \frac{1}{- 8}$
P = −0.125 D
A long-sighted person has a near point of 6 m. What power of lens is needed to correct this?
u = 0.25 m (normal near point), v = −6 m (virtual image at the uncorrected near point)
$\frac{1}{f} = \frac{1}{0.25} - \frac{1}{6} = 3.83$
P = 3.8 D
True or False?
A short-sighted person needs a converging lens to correct their vision.
False.
Short-sightedness is corrected with a diverging lens, which causes light rays to diverge before entering the eye so the image forms on the retina instead of in front of it.
Define astigmatism.
A defect of vision caused by an irregularly shaped (non-spherical) cornea, which gives different focal lengths for different planes of vision.
What is meant by the cylindrical power (CYL) of an astigmatism?
The difference in power between the two points of focus of the two planes of light. The greater the difference, the greater the astigmatism. Measured in dioptres (D).
What three pieces of information does a prescription for astigmatism give for each eye?
- Sphere (SPH) — the focal power needed to correct long- or short-sightedness
- Cylinder (CYL) — the cylindrical power needed to correct the astigmatism
- Axis angle — the angle needed by the lens to correct the astigmatism
How does the axis angle of a prescription relate to the angle of astigmatism on the cornea?
The axis angle of the correcting lens is perpendicular (90°) to the angle of astigmatism on the cornea.

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