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Optics
Optics is the study of light. Light allows us to see when it reflects off of objects
around us and into our eyes. It usually travels in a straight line, but today we will
learn how to bend it with mirrors and lenses.
Bending Light
The mirrors you usually use are flat, so everything looks normal in them. Have you
ever seen a funhouse mirror? Those are curved, which makes the reflections look
weird. Curved mirrors come in two types: concave (curves inwards like a cave) and
convex (curves outward).
Metal spoons can act like mirrors:
 Look at yourself in the concave side of the spoon – what do you notice about
your reflection? What happens when you bring this side of the spoon very close
to your eye?
 Now look in the convex side of the spoon. Is your reflection the same as in the
concave side?
Lenses: Focal Points and Images
We will be spending the rest of the day working with lenses. Lenses are pieces of
glass that are curved. Just like curved mirrors, they make light bend in funny ways.
However, the light passes through the lens rather than bouncing off as it does with a
mirror.
Magnifying glasses, such as the ones we will be working with today, use lenses to
magnify objects.
 Try using the magnifying glasses to look at different objects.
 Hold it at different distances from your eye and from the object you are looking
at. What happens?
We can use diagrams called “ray diagrams” to show how the light bends as it
moves through a lens. Lenses have something called a “focal point.” Light rays that
are moving parallel (in the same direction) to each other through a lens will be bent
so they all go through the focal point.
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focal point
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You can find the focal point of your lenses by focusing light from a window onto a
piece of paper.
 Hold up the paper so that it is facing the window, and put the lens between the
window and the paper.
 Move the lens back and forth from the paper until you form a clear image. When
the image is clear, the paper is at the focal point.
 You may notice the image is upside-down. You will find out why in a minute!
Ray Diagrams
Light rays from objects at different distances from a lens bend in different
ways. When the object is at the focal point, the light rays coming out of the lens are
parallel.
When the object is past the focal point, the light rays come together on the opposite
side of the lens to form an “image” of the object that is bigger than the object itself
and upside-down. (The image you formed from the window before was from an
object past the focal point. That is why it was upside-down!)
When the object is closer to the lens than the focal point, the light rays spread out on
the opposite side of the lens.
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Another way to find the focal point is to hold the lens away from your eyes and
moving it back and forth from the object you are looking at.
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How far are you holding the lens away from the object when you can see it most
clearly? That is the distance to the focal point.
Two Lenses
The light that passes through one lens can be bent again through another lens.
If you use two lenses, you can change the image that you see.
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Look at something far away through the smaller lens on your magnifying glass.
Once you get it in focus, take another magnifying glass and put the larger lens
behind the small one you are already looking through. If you can’t reach far
enough, get your mentor to hold it or mount the magnifying glass on a cup to hold
it up.
Try to position the lenses so you are looking through both. If you get them the
right distance apart, you should be able to see an image that is clear and rightside-up.
Now try holding the small lens right up to your eye.
Hold the large lens on the other side of it, and move it back and forth until the
image you see is clear.
The image should be upside-down and slightly magnified.
Congratulations! You have made a telescope. Real telescopes use lenses and
mirrors to flip the image back right-side-up.
Now try holding one large lens close to your eye. Hold another large lens behind
it, and move it different distance from your eye until you can see clearly through
both lenses at once.
If you keep the lenses the same distance apart, you can move them back and forth
from your eye and still see clearly through them!
Is the image you see right-side-up or upside-down? Does that make sense with the
ray diagram below?
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The distance between the two lenses is equal to two times the focal length.
Looking at the diagram, why do you think this is true?
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Questions
1. What is the difference between convex and concave mirrors? Why
does your reflection look strange when you look at yourself in a convex or
concave mirror?
2. When you were looking through just the large lens, how far away from
the lens did the object have to be for you to see it clearly? What about
when you were looking through the small lens? What does that say about
their focal points?
3. What are some objects you can think of that use lenses (other than
magnifying glasses)?