Physics of the Eye (AQA A Level Physics)

A system of lenses consists of a converging lens and a diverging lens in contact.

The magnitude of the power of the converging lens is 9.4 D and the magnitude of the power of the diverging lens is 4.2 D.

Calculate the power of this system of lenses.

Distinguish between the nature of the real and virtual images produced by a converging lens.

A converging lens has a focal length of 0.15 m. An object is initially placed 0.4 m in front of the lens and an image is formed. The object is then moved to a point 0.2 m in front of the lens.

Compare the properties of the two different images produced.

When the object is placed 0.2 m in front of the lens an image is formed 0.05 m behind the lens.

Calculate the magnitude of the magnification of the lens.

A student is trying to read a stop sign on the other side of a road but it is completely blurry as shown in Figure 1.

Figure 1

Explain the vision defect the student has and how it can be corrected.

A different student is looking at the same sign from the same distance away. They notice that the letters on the sign are blurry but only in the vertical direction as shown in Figure 2.

Figure 2

Explain this defect of vision.

Which ray diagram in Figure 3 shows the defect of vision of the student in part (b)?

Explain your answer.

Figure 3

Table 1 shows some prescriptions for glasses to correct a defect of vision in one eye.

Table 1

Which would be a correct lens prescription for the student in part (b)?

Explain your answer.

Which would be a correct lens prescription for a person with hypermetropia and astigmatism?

Electrophoretic screens are used in handheld electronic devices.

The screen contains individual squares known as pixels. Pixels can be changed independently from light to dark to create the shapes of letters and numbers. An external light source is needed in order to read the screen.

Figure 1 shows a letter formed by three electrophoretic screens that have different pixel line densities. Pixel line density is the number of pixels along a 1.0 cm length of the screen.

Figure 1

A particular screen is designed so that two dark pixels separated by one light pixel cannot be resolved as separate images by the eye when viewed from a distance of 0.50 m.

Determine, in pixels per cm, the minimum pixel line density required for this screen.

• typical diameter of cones in a human eye at the fovea = 1.5 μm

• typical length of the human eye = 21 mm

On a different electrophoretic screen, two dark pixels separated by one light pixel can just be resolved at a particular distance when the external light source is bright.

Explain why these pixels cannot be resolved at the same distance when the intensity of the external light source is reduced.

Figure 2 shows the spectral response of the three different types of cone in a human eye.

Figure 2

The eye is illuminated by light of wavelength 603 nm.

Show that the response of a red-sensitive cone is approximately double the response of a green-sensitive cone.

Other types of screen use blue, green and red pixels to produce coloured images.

Table 1 shows the wavelength of the light emitted by each pixel when it is turned on.

Table 1

On one screen, the blue pixels are turned off.

When the green pixels and the red pixels are turned on, they emit light with the same intensity. A human eye that has the spectral response shown in Figure 2 responds to this light.

Determine, in nm, the single wavelength of light that will produce the same response in the same human eye as the light emitted from the green and red pixels.