The Lens Lab

The Lens Lab

A lens is a shaped piece of glass or plastic that bends light by refraction. Light refracts once as it enters the lens and again as it leaves, and the two bends together form an image. Glasses, cameras, projectors and magnifying glasses all rely on this. So does the fact that the same converging lens can make an image you can project onto a wall, or one you can only see by looking through it, depending on nothing more than how far away you hold the object.

Converging and diverging

Two shapes, two opposite jobs. Everything else in this module follows from which one you are holding:

Which lens is it?

A lens is held up towards a distant window, and a small, sharp, upside-down picture of the window appears on a card held behind it. Which kind of lens is it, and how do you know?

• Converging: the picture landed on a card, so the rays really met there, and only a converging lens can bring rays together to an actual meeting point
• Diverging: the light spread out from the lens to make a picture bigger than the lens itself
• Either could do this, depending on how far away the card is held
• Neither: to form a picture on a card you would need a curved mirror rather than a lens

The words for it

Six terms. The two image types are the ones every exam question turns on, so read those twice:

What does each observation show?

How the diagram is drawn

A converging-lens ray diagram is built from just two rays leaving the top of the object. Where they cross is where the top of the image sits. • A ray travelling parallel to the principal axis is refracted so that it passes through the principal focus on the far side. • A ray heading for the centre of the lens carries straight on, undeviated, because the two surfaces are parallel there. Two rays are enough. A third is sometimes drawn as a check, and if it does not pass through the same point, something has gone wrong with the first two.

Label the ray diagram

This is a converging lens forming a real image. Drag each label onto the correct part.

The magnifying glass

Take the same converging lens that made a picture on a card, and hold it close to a stamp instead, nearer than its focal length. Now something different happens. The rays leaving the lens are still bent inwards, but not enough to cross before they reach your eye. They arrive spreading apart, and your eye traces them back to a point behind the stamp. That is where you see the image: enlarged, the right way up, and impossible to catch on anything. Move the lens further away and, as the object passes the focal point, the image flips: it becomes real, inverted and projectable. One lens, two behaviours, decided entirely by whether the object is inside or outside the focal length.

Why can you not catch it?

You hold a card behind a magnifying glass to try to project the enlarged image of a stamp onto it. Nothing appears. Why not?

• The refracted rays are still spreading apart when they leave the lens, so they never cross anywhere. The image is where they only appear to come from, which is behind the object, and no card can be put there
• The image is too faint and spread out to show up on a card
• The card is on the wrong side: it should be held in front of the lens instead
• The card is not exactly at the principal focus, and moving it there would work

Work out the magnification

An object is 2 cm tall. A lens forms an image of it 6 cm tall. What is the magnification? (It has no units: give the number.)

Find the image height

A lens gives a magnification of 3, and the object is 4 cm tall. How tall is the image, in centimetres (cm)?

Which are true?

Select ALL THREE statements that are TRUE.

• A diverging lens can never produce an image you could catch on a screen
• A real image is always inverted, which is why a projector has to be fed its picture the other way up
• Magnification has no units, because it is one length divided by another
• A virtual image can be projected if the screen is placed behind the lens instead of in front
• A converging lens always produces a real image
• A magnification of 0.5 means the image is twice as tall as the object

Lab summary

A _____ (convex) lens is thicker in the middle and brings parallel rays together at the principal focus. A real image forms where rays actually _____, can be shown on a screen and is inverted, while a _____ image forms where rays only appear to come from and cannot be projected. Magnification = image height ÷ _____ height, and has no units.

Real or virtual?

Exam practice. In about 50 words, explain the difference between a real and a virtual image, and describe a simple test you could do in a lab to find out which kind a lens is producing. Include:

• what the refracted rays are doing in each case
• the test itself, and what result would tell you which image you have
• which way up each kind of image appears