U2.3 Lenses

Question 1

What is a convex lens

Answer 1

They are lenses that are thicker in the middle. they cause parallel rays of light to converge towards a point on passing through them

Question 2

What is a concave lens

Answer 2

A lens that is thicker at the edges.

Question 3

What is the principal axis

Answer 3

The line joining the centres of curvature of the two faces of the lens.

Question 4

What is the principal focus of a convex lens

Answer 4

The point on the principal axis towards which all rays parallel to the principal axis converge after passing through the lens.

Question 5

What is the principal focus of a concave lens

Answer 5

The point in the principal axis from which all rays parallel to the principal axis appear to diverge after passing through the lens

Question 6

What is the focal length

Answer 6

The distance from the centre of the lens to the principal focus

Question 7

What is the definition of a virtual image

Answer 7

It cannot be formed on a screen and the rays of light do not actually meet at the image.

Question 8

What is the definition of a real image

Answer 8

It can be formed on a screen and the rays of light meet at the image

Question 9

What is the lens formula

Answer 9

1/u + 1/v = 1/f

U= object distance
V= image distance
F= focal length

Question 10

What is the equation for magnification

Answer 10

Magnification = hight of image / hight of object

m = h_i / h_o

Question 11

What is the other equation for magnification

Answer 11

Magnification = image distance / object distance

m = v / u

Question 12

What is the equation for lens power

Answer 12

Power of a lens (in D) = 1 / focal length (in m)

Power = 1/f

Question 13

What is lens power measured in

Answer 13

Dioptre (D)

Question 14

What are the characteristics of a normal eye

Answer 14

-Far point is infinity

-Near point is 25cm

Question 15

What is myopia (short sight)

Answer 15

Near objects can be seen clearly (i.e. 25cm near point)

Distant objects are blurred due to image being formed in front of the retina

Question 16

What is the cause of myopia

Answer 16

The eyeball is to long for the focal length of the lens or the lens is to strong for the length of the eyeball

Question 17

How is myopia corrected

Answer 17

A concave lens is used ti produce a virtual image at the eyes far point this image then acts as an object in the eye

Question 18

What is hypermetropia (long sight)

Answer 18

Distant objects can be seen clearly (i.e. the far point is infinity)

Close objects are blurred because the image is formed behind the retina. Hence the near point is further from the eye.

Question 19

What is the cause of hypermetropia

Answer 19

They eye is to short for the focal length or the lens is to weak for the length of the eyeball.

Question 20

How is hypermetropia corrected

Answer 20

A convex lens is used to produce a virtual image at the eyes near point. This image then acts as an object for the lens in the eye