Option - Chapter 1

Question 1

What does the Cornea do in the eye?

Answer 1

The Cornea is a protective transparent layer at the front of the eye. It has a fixed curvature and therefore a fixed focus lens.

Question 2

What do the Ciliary muscles do?

Answer 2

They change the thickness of the eye lens, therefore altering the optical power of the eye lens.

Question 3

Where are the Ciliary muscles fibres located? and how do the alter to view near and far objects?

Answer 3

The fibres lie along the concentric circles around the rim of the eye lens. To view near objects the fibres shorten making the eye lens thicker and more powerful. For far objects, the fibres relax allowing the eye lens to become thinner and less powerful.

Question 4

What does the iris do and where is it located?

Answer 4

The iris is located between the cornea and the eye lens and it controls the amount of light which enters the eye.

Question 5

What does the iris consist of and how do the constituent parts change in bright and dim light?

Answer 5

The iris consists of concentric and radial muscle fibres around the circular hole of the pupil. In bight light, the concentric fibres contract and the radial fibres relax so the iris expands making the pupil narrower. In the dim light, the concentric fibres relax and the radial fibres contract so the iris contracts, making the pupil dilate.

Question 6

What is the retina made up of?

Answer 6

The retina is a layer of light-sensitive cells at the back of the cell at the back of the eye. Made up of two types of cells, cones and rods.

Question 7

In the retina where are mostly cones located and where are mostly rods located.

Answer 7

Mostly cones - in the Fovea, which is near the principal axis of the eye lens
Mostly rods - near the periphery of the retina (edge)

Question 8

How many photons need to be absorbed to trigger a rod?

Answer 8

10

Question 9

What is an after image?

Answer 9

It is even after a very bright image disappears, this is because the cones can take up to a second to regain normal sensitivity.

Question 10

What is the resolution of the eye determined by?

Answer 10

It is determined by the size and closeness of the retina cells.

Question 11

How far does a diffracted image need to be to form two separate images?

Answer 11

The image needs to be separated by at least two retinal cells to be resolved.

Question 12

What is the formula for angular separation and separation of the image centres?

Answer 12

Angular separation - radians=d/u d= separation u= distance from the eye
Image centres - y=radiansv v= distance from the eye lens to the retina.

Question 13

What is a converging and diverging lens?

Answer 13

Converging - brings parallel rays to a point, this point is called the principal focus or the focal point.
Diverging - causes parallel rays to spread out, the rays appear to come from is the principal focus.

Question 14

How do you draw a ray diagram?

Answer 14

1, By drawing a parallel ray to the axis which is then refracted through F on the other side.
2, By drawing a ray through the lens at its centre without any change of direction after.
3, By drawing a ray through F before the lens, so it is refracted by the lens parallel to the axis.

Question 15

If the object is beyond two F what happens to the image formed? For a converging lens

Answer 15

A real image is formed between F and two F and the image is smaller than the object.

Question 16

If the object is between 2F and F what happens to the image formed? For a converging lens

Answer 16

A real image is formed which is magnified and inverted

Question 17

If the object is sitting on 2F whats happens to the image formed? For a converging lens

Answer 17

A real image of the same size which is inverted.

Question 18

If the object is sitting less than F what happens to the image formed? For a converging lens

Answer 18

A virtual image is formed which is bigger than the object and is upright.

Question 19

When using the lens equation what is important?

Answer 19

Real images are given positive values and virtual images negative ones.

Question 20

What is a lens defined as? And what is its unit

Answer 20

1 over its focal length in metres and the unit is dioptres

Question 21

What values of power are converging and diverging lens given?

Answer 21

Converging are positive and diverging and negative.

Question 22

What is the range of site for a normal person eye?

Answer 22

25 cm to infinity.

Question 23

What are Myopia and Hypermetropia and what are they caused by?

Answer 23

Myopia or short sight is when an eye cant focus on distant objects, this is because the eye muscles cannot make the lens thin enough or the eye is too long.
Hypermetropia or long sight is when an eye cannot focus on nearby objects, this is because eye muscles cannot make the lens thick enough or the eyeball is too short.

Question 24

How are Myopia and Hypermetropia corrected?

Answer 24

Myopia is corrected by a diverging lens which has a focal length equal to the distance from the eye to the uncorrected far point.
Hypermetropia is corrected with a converging lens which has a focal length that makes an object placed 25 cm from the eye appear as if it is at the uncorrected near point.

Question 25

What is an Astigmatism and how is it corrected?

Answer 25

An Astigmatism is a sight defeat in which objects are seen to be in sharper focus in one direction than any other. Astigmatism is caused by an uneven curvature of the cornea. To correct astigmatism it requires a lens with a cylindrical-shaped surface, orientated so that it compensates for the uneven curvature of the cornea.

Question 26

What is a suitable test for an Astigmatism?

Answer 26

A test is to observe two sets of parallel lines on a card that are perpendicular to each other if one set stands out more than any other the eye has an Astigmatism.