Lenses

There are two different types of lenses.

Convex lens Concave Lens

Copy the diagram of each lens into your book. You should have an example of each type of lens, look through each lens at objects that are close and far away. Record what you see in your book. How does this change if the lens is moved away from the eye?

Lenses

To find out why you need to collect some equipment.

A ray box, a power supply, a sheet of plain paper and two lenses. Your teacher will show you how to work the power supply.

Start with the convex lens.

Convex lenses are also known as converging lenses

Concave lenses are also known as diverging lens

Lenses

Place the lens in the centre of the paper. Use the ray box to produce 3 parallel rays of light which hit the lens. Make sure one of them hits the centre of the lens.

Incident rays

What happens to the rays when they pass through the lens?

Produce a good diagram of what you see in your books and explain why concave lenses are also called converging lenses.

Lenses

Incident rays(parallel)

F

Principal focus

f(focal length)

This is what you should have seen.

Did you notice that the light rays bend on entering the lens and leaving the lens?Make sure you know what the princpal focus is and how this allows us to measure the focal length.

What was the focal length of your lens?

Lenses

Principal focus

Incident rays of light are parallel

Another convex Lens

Lenses

Concave Lens – know do the same for the convex lens

Incident rays(parallel)

What happens to the rays when they pass through the lens?

Produce a good diagram of what you see in your books and explain why concave lenses are also called diverging lenses.

Lenses

Incident rays(parallel)

(focal length)

F

f

The light rays are spread out or DIVERGED by this lens.

Parallel rays are diverged so that they appear to come from the principal focus (F) of the concave lens.

Objects always look smaller through a concave lens.

Did you notice that the light rays bend on entering the lens and leaving the lens?Make sure you know what the principal focus and focal length are for a concave lens. Can you think of a way of measuring the focal

length of your diverging lens? (Hint it involves drawing)l

LensesIncident rays are parallel

Rays leaving the lens are divergingThe black lines show where the diverging light appears to have come from the Prinicpal Focus

Lenses

Forming Images

The rays of light from the car are converged by the lens, to form an image on the screen.

The image is inverted (upside down) and real (the rays of light pass through the lens and an image is formed on the screen) and diminished (smaller than the object).

Screen

Lenses

Film/screen

Image: Real, Inverted and diminished

Object

In a camera the image is smaller than the object we say the image is diminished

Cameras

Lenses

Insert photograph hereProjectors

The image is inverted, real (its on a screen ) and magnified (larger than the object)

Magnification = height of imageheight of object

Lenses

Question 1. – Copy out and complete the following

a) A convex lens _____ rays of light, but a concave lens _____ rays of light.

b) Parallel rays of light are refracted by a convex lens to a point called the _____ ____.

The focal length is the distance from the _____ _____ to the _____ of the lens.

c) A fat convex lens is a _____ lens, with a _____ focal length.

d) A real image can be formed on a _______e) The image in a concave lens is always _______ than

the ______.Centre, converges, diverges, focus, object, principal, screen, short, smaller, strong

Answer

Lenses

Question 1. – Copy out and complete the following

a) A convex lens converges rays of light, but a concave lens diverges rays of light.

b) Parallel rays of light are refracted by a convex lens to a point called the principal focus.

The focal length is the distance from the principal focus to the center of the lens.

c) A fat convex lens is a strong lens, with a short focal length.

d) A real image can be formed on a screen.e) The image in a concave lens is always smaller than

the object.

Got something wrong ask your teacher!!

Lenses

Question 2 – copy and complete the ray diagrams

F

A B

C D

Answer

LensesQuestion 2 - answers

F

A B

C D

Did you remember to show the light bending on entering and leaving the lens?

Got something wrong ask your teacher!!

Lenses

That’s all for now folks

but

You need to check through the work from today at home

Next lesson: you will be using what you have learnt today to predict the size and position of an image and then checking your prediction.

Insert picture here