# Labs from YWI 91 : Invisible Rays.

Dr. Jim Sweitzer
CARA Yerkes Winter Institute, December 1991

Only part of this is meant to be given to the students; there is a Teacher's Guide at the end.

## Introduction

This activity is designed to help you appreciate the fact that there is light from the sun and light bulbs that is invisible to our eyes, but nevertheless exists. To discover this, your team will break into two groups and investigate the existence of this invisible light two ways. After about 25 minutes you will switch with the other group. The source of this invisible radiation will depend on whether it is sunny or not when you do the activity. If it is sunny, we will use a beam of sunlight to do the lab investigations. If it is cloudy, we will use the light from a slide projector.

## Lab Investigations:

1. Put your hand into the beam of light. Does it get warm? ______.

Next, put the glass filter (which lets visible light pass through it) marked "Filter A" in the light path before your hand. What happens? _______________________________________

What happens when we put "Filter B" (which is opaque to visible light and will be used on the camera to take pictures) in the beam of light?__________________________________________

Is the heat, then due to visible light? ______________________

2. If the heat is not visible light, is it a type of invisible light, or is it something totally different from light? One way to investigate this question is by seeing if the heat rays have some of the same properties as visible light.

Using the mirror and prism, investigate the heat rays to see if they have the following two properties:

Reflection (we all know what this is): Do the heat rays reflect like visible light?_____________

Refraction (This is the property whereby light bends when it passes from one medium to another. The most familiar example is a lens that concentrates light. The other example you might be familiar with is a prism. Not only does the prism deflect light, it also breaks it into different colors. For now, just use the prism to refract the light.): Do the heat rays refract like visible light? ________________________

Do you think the heat rays are a type of light?_____________

3. Finally, if the heat rays are a type of light, they should have their own wavelength. Recall that each particle of visible light has its own particular wavelength. Every color of light you see corresponds to a different wavelength. Let's see if the invisible heat radiation has a wavelength and how it is different from the wavelengths of visible light. To do this we will use the prism and the sheets of liquid crystals.

First, note that the liquid crystals respond to the heat of your hand by changing color. The color they change into has nothing to do with the "color" of the heat radiation, rather it is a property of the liquid crystals. We will use the liquid crystals simply as way to detect heat.

Using the prism, the liquid crystals, and Filter A, figure out if the heat rays are on the red side or the blue side of the visible spectrum. Answer: _____________________________________

## Photographic Investigations

You will be taking "Filter B" and placing it on the camera to see if you can take pictures with the heat rays. Your assignment is to take 12 pictures with the camera. It is loaded with special film that records heat rays as well as visible light. The filter is placed over the lens so that only the invisible rays can pass through. This makes focusing and aiming the camera very difficult. To focus, measure the distance to your subject and then line up the red dot across from the distance on the lens.

To point, you will just have to aim and hope the camera is directed where you want.

Here are some exposures that worked for me:

Outdoors on a sunny day, I used 1/250 second at f2.8. On a cloudy day, I used 1/30 second at f4.

With a flash I got the best results with a guide number of 40. To use a guide number, you divide the distance to your subject into 40. The result is the f-stop you should use. Make sure that the camera is set at 1/60 second, the camera is focused properly, and the flash is set on "m."

Record your photographs in the table below. We'll develop the film this afternoon so you can see the pictures tonight.

Exposure # Lighting Shutter speed F-stop Subject

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If heat rays can record photos, do you think that heat rays are a type of light?____________________.

Where do the heat rays come from in the different types of pictures?

Flash Pictures:_________________________________________

Outdoor Pictures?:_____________________________________

Here's the final question. Remember the red spot on the barrel of the camera lens? It was to allow you to properly focus this light. Now, use the following three facts to decide whether this invisible light is redder than red or bluer than blue.

Fact 1: As you focus a camera lens on something farther and farther away, you must move the lens closer and closer to the film.

Fact 2: Red light bends less than blue light through a lens.

Fact 3: To focus the invisible light from an object, you had to move the lens out slightly from where you would have had it focused for visible light.

So, which side of the spectrum is the invisible light on? __________________________________________________

## Teacher's Guide:

### Objectives:

1. To demonstrate the existence of infrared light
2. To prove that infrared radiation is actually light
3. To prove that infrared radiation is redder than red.

### Tools or Demonstrations:

1. Coelostat to give 1" beam of sunlight
2. Infrared film
3. 35mm camera
4. Wratten 37B filter
5. IR absorbing glass
6. Slide projector with IR glass removed
7. Liquid crystals for various temperature ranges
8. Lenses
9. Thermometers
10. Mirror
11. Various stands
12. Prism

### Outline:

```I. How do we know it exists and is different from visible light?
A. Put hand in beam to feel it.
B. Put it on to liquid crystals.
C. Block with IR filter.
D. Block visible light with 37B filter (this may not work with
the sun without melting the filter, so be careful.)
2. How do we know it's light?
A. Reflect it off of mirror.
B. Refract it with prism or lens.
C. Polarize it with polarizers?
D. Photograph with it using 37B.
1. Couple photoflood pictures
2. Try to photo it off of prism red side
3. Go outside
3. How do we know it's redder than red?
A. If we have sunlight, try using prism and thermometers.
B. If no sun?
C. Argue from design of lens?????
```

Important Disclaimers and Caveats: