Lenses
AIM
The aim of these experiments is to help you understand something about lenses.
YOU WILL NEED
A convex lens (focal length +10cm), a concave lens (focal length -10cm), a cylindrical convex lens, a cylindrical concave lens, a ray box with a three slit mask, a convex lens for use in the ray box to produce a parallel beam of light (if needed), a suitable power supply, a sheet of white paper.
UNDER NO CIRCUMSTANCES USE ANY OF THE LENSES TO LOOK AT THE SUN OR ANY OTHER BRIGHT SOURCE OF LIGHT. FAILURE TO OBEY THIS INSTRUCTION COULD LEAD TO SERIOUS EYE DAMAGE.
WHAT TO DO
Copy the labelled diagrams from the top of this sheet into your books or onto your paper.
The first section of this experiment is to use the lenses to look through and to attempt to produce an image. The second section goes on to investigate the effect of the cylindrical lenses on a beam of light.
Section 1
Using the convex lens.
1. Hold the lens about 5cm from your eye and look at:
(i) an object about 20cm from the lens
(ii) an object about 4m from the lens
Describe what you see.
Is the image seen through the lens:
(a) blurred or clear
(b) upside down or the right way up
(c) bigger or smaller than the object
(d) coloured or in black and white
2. Now hold the lens at arms length and use it to look at:
(i) an object about 5 cm from the lens
(ii) an object about 4m from the lens
Describe what you see.
Is the image seen through the lens:
(a) blurred or clear
(b) upside down or the right way up
(c) bigger or smaller than the object
(d) coloured or in black and white
3. Hold the lens about 15 cm from a wall and see if you can form an image of the window of the lab on the wall.
Can you get an image at all?
Is the image of the window:
(a) blurred or clear
(b) upside down or the right way up
(c) bigger or smaller than the object
(d) coloured or in black and white
Repeat all these experiments using the concave lens.
Section 2
1. Place the sheet of white paper on the bench, put the ray box on it and place the convex cylindrical lens in front of it. Switch on the power supply so that the ray box gives three beams of light which should be parallel as shown in the following diagram. If they are not parallel to each other adjust your ray box or its internal lens.
Draw round the lens, mark the three incoming rays and draw in the rays after they emerge from the lens to show the effect of the lens on a beam of parallel light. Try changing the distance from the ray box to the lens to see if that makes any difference to the emerging rays.
Repeat the experiment using the cylindrical concave lens.
2. Remove the internal lens of the ray box, or adjust the ray box, so that the three rays coming from it are spreading out (diverging) and put the cylindrical convex lens in the path of the rays (as shown in the following diagram).
Once again draw round the lens, mark the three incoming rays and draw in the rays after they emerge from the lens to show the effect of the lens on a diverging beam light. Try changing the distance from the ray box to the lens to see if that makes any difference to the emerging rays.
Repeat the experiment using the cylindrical concave lens.
Questions
1. What difference do you think it would make to the results of the experiments in section 1 (3) and section 2(1) if the convex lens was fatter in the middle (that is if the faces of the lens were more sharply curved)?
2. Using your results from section 1 suggest with reasons what type of lens is in the human eye.
3. Using your results from section 1 suggest what type of lens would be found in a spy hole in a door to give a wide field of view.
4. Use your results in section 2 to explain the results of section 1(3) when you use the two different types of lens.
5. Why do you think it is so dangerous to use a convex lens to look at a bright source of light such as the Sun?