1. Light enables us to see.

Without light, we would have no sight. In order to see an object, light must come from the object and travel into our eyes, where it stimulates special nerve receptors on the retina at the back of the eyeball. How does light “come from an object”? If the object is luminous, like the sun or the filament of a light bulb, it produces light that radiates outward in all directions, allowing anyone in a certain line of sight to be able to see it. Most objects, however, are non-luminous. They do not produce their own light, but they do reflect some of the light that strikes them. When light strikes most objects, it is not reflected in just a certain direction, but is scattered in all directions. This allows the object to be seen from different viewing positions. A person located anywhere in the pathway of the scattered light will receive light from the object and thus be able to see the object. 

If there is no light to illuminate our surroundings, we cannot see any of the objects around us. In simplest terms: No light, no sight.

2. Light travels in straight lines until it strikes an object or surface.

There is a lot of evidence that light travels in straight lines. Sometimes, when sunlight passes through clouds, we can actually see straight lines of light streaming toward the earth. Similarly, the beam of a spotlight or a laser in the night sky also appears to follow a straight path. In the science laboratory, a series of pinholes must be lined up for light to pass through them and illuminate a surface beyond. As another example, a hose or tube must be stretched very straight in order for light entering one end to emerge from the other end. Evidence that light travels in straight lines is also provided by the fact that shadows are the same shape as the objects that cast them.
When light strikes an object, the direction of its straight-line path may change. The fact that a beam of light can be reflected from a mirror or converged by a lens are examples of how light may change its direction upon striking an object or surface. Light may also bend around the edges of an object or as it passes through a small opening; this is called diffraction and was not covered in this workshop. Diffraction is the reason the edges of many shadows appear fuzzy.

3. Light from a source radiates outward in all directions from every point on the source.

Every point on the surface of a light source can be thought of as a point source of light.  From every point, light radiates outward in straight lines in all directions.  This explains why a pinhole, placed between a light source and a screen, causes an inverted reproduction of the source to be seen on the screen. 

4. If an object is placed between a light source and an illuminated surface, a shadow is formed on the surface.

A shadow is a dark figure cast upon a surface by an object that intercepts the rays from a light source. 

The properties of a shadow are linked to the nature and position of the light source, the shape and characteristics of the object that casts the shadow, and the location of the screen relative to the object.  

Shadows resemble the shape, but not necessarily the size, of the object that makes them. However, an object may create shadows of different shapes depending on its orientation or the orientation of the screen. For example, a cylinder may create a shadow that is a rectangle or a shadow that is a circle. The shape of the shadow will always be the same as the cross-section of the object that lies between the light source and the surface.  

A couple of things determine the size of a shadow. A shadow gets bigger as the distance between the object and the surface on which the shadow is formed increases.  

The angle of the light source relative to the object and the screen also determines the size of the shadow. For example, when the sun is almost directly overhead in the middle of the day, your shadow is very short. When the sun is about to set in the late afternoon, your shadow is very long.

Transparent, translucent, and opaque objects can all create shadows. An opaque object casts a shadow that is totally dark because it blocks all of the light rays falling on it; no light passes directly behind the object. (Actually, shadows cast by opaque objects are rarely totally dark because light reflected from walls and other objects may fall onto the surface where the shadow is.)

A transparent or translucent object casts a lighter shadow because it blocks only some of the light rays falling onto it. The other light rays pass through the object and onto the surface beyond. Thus the shadow is an area of reduced illumination (compared to the fully illuminated surrounding area).

Some shadows, like the one below, have an umbra (dark area) and a penumbra (lighter surrounding area). The dark shadow, or umbra, is formed where no light from the source can reach the surface. The lighter shadow surrounding the upper portion of the umbra is produced where light from a part of the light source can the screen, but light from the other part of the source is blocked. For example, the lighter shadow below the umbra is produced where light from the lower portion of the light source can reach the screen, but light from the upper portion of the source is blocked.

A solar eclipse is a good example of the formation of an umbra and penumbra. A solar eclipse occurs when the moon passes between the sun and the earth, casting a shadow on the earth’s surface. The shadow formed on earth has both an umbra and penumbra. If you are located in the umbra of the shadow produced during a solar eclipse, you will not be able to see the sun at all. But if your location on earth lies within the penumbra, you will be able to see part of the sun. It is light from this edge that reaches your location on earth and prevents you from being in total darkness.

5. An image is formed when multiple light rays from every point on an object converge to a single set of corresponding points on a screen or other similar surface.

When we look at an object such as a tree, all of the light from the top point on the tree that enters our eyes must converge to a single point on the retina in order for us to clearly see the top of the tree.  At the same time, all light entering our eyes from every other point on the tree must converge to a corresponding spot on our retina in order to produce a clear and complete retinal image of the tree.  Suppose that the light from the top of the tree does not converge to a single point on our retina, but instead strikes the retina at several different places (or is spread out over a slightly larger area of the retina). Then the top of the tree appears very blurred. And if the light is spread over an even larger area of the retina, we may not be able to distinguish the top of the tree at all. Images are produced in a camera in much the same way they are produced by the human eye. Light rays from the object being photographed form an image on the film inside the camera. If the light rays do not converge exactly at the film surface, then the image on the photograph will be blurred. When we focus the camera, we are actually adjusting the lens to make the light rays converge exactly at the film.

6. Light is a form of energy.
Energy is needed to produce light. Every light source is really an energy converter, changing some form of energy into light. As examples:

Nuclear Energy SUN Light

Chemical Energy CANDLE Light

Electrical energy LIGHT BULB Light

Energy is gained by anything that absorbs light. Light can make things move, can be used to produce electricity, and can warm things up. When light shines on the blades of a radiometer, they begin to spin. When light shines on a solar cell in a circuit, electrical current begins to flow and can sound a buzzer, light a bulb, or even move a car. When light from the sun shines on the earth, it warms the earth’s surface and the air surrounding it. Light makes things happen. Light is energy.

7. What we usually call “light” is actually the narrow range of radiant energy that can stimulate special receptors in the human eye which permit vision.

What the human eye sees as light, such as sunlight or the glow from a candle, is actually radiant energy (also referred to as electromagnetic radiation). There is a broad spectrum of radiant energy, a very small range of which humans can actually see. 

When radiant energy within this range enters the eye, it stimulates receptors along the inside back wall of the eyeball, which then transmit information through the optic nerve to the brain. If radiant energy outside the visible range enters the eye, it does not stimulate the receptors in the eye and there is only darkness.

Radiant energy outside the visible range affects the human body in other ways. Ultraviolet radiation penetrates the outer layers of the skin and causes sunburn, while x-rays can pass completely through body tissue but are absorbed by bones which makes them useful for taking pictures. Infrared radiation, on the other side of the visible spectrum, creates a sensation of warmth upon striking the human skin. For this reason, infrared radiation is often referred to as heat radiation; anything that gives off heat is really emitting infrared radiation. As an example, an incandescent bulb feels hot because it gives off infrared radiation as well as visible light. A fluorescent light bulb, on the other hand, is much cooler to the touch because it gives off very little radiation in the infrared range.

8. To explain how light is produced, scientists use a model which considers light as particles, or photons.

NOTE: Scientists sometimes use different models to explain different aspects of something. They commonly use two different models to help us understand light (and other forms of radiant energy). To explain how light is emitted and absorbed, scientists use a model that considers light as particles, called photons. To explain how light travels, scientists use a different model, which considers light as waves.

According to the particle model, radiant energy consists of bundles of energy, or photons, that behave very much like particles. Photons of different types of radiant energy have different amounts of energy. Consider an example mentioned earlier. Ultraviolet radiation consists of photons that have only enough energy to penetrate the skin. X-ray photons have more energy, and can penetrate the skin and body tissue. Gamma ray photons have even more energy and can pass through almost a foot of concrete before being completely absorbed.

Even within the range of visible light there are differences in the energy of the photons. A photon of red light, for example, has less energy than a photon of green or blue light.

9. The particle, or photon, model provides a single explanation for how all light is produced.

Light comes from a wide variety of sources: a candle, the sun, a light bulb, a Lumastick®, a firefly, a sparking Wintergreen mint, etc. These sources seem so different – as does the light that each produces. Yet the production of light by all of these sources can be explained using the photon model.

When an object absorbs energy, electrons in the atoms of the object become energized and jump to higher-than-normal energy levels. This process is referred to as “electron excitation”.

Excited electrons do not remain at these higher energy levels, but return to lower levels by releasing the excess energy as radiant energy. Every time an excited electron drops from a higher to a lower energy level, it emits a photon of radiant energy that has the same amount of energy as the difference between the two levels.

In a light bulb, electrons in billions of atoms of the filament are simultaneously and continuously emitting photons in this way.

10. Objects constantly absorb energy from their surroundings and emit radiation.

We are unaware that this is happening with most objects because the photons they emit have such little energy that they are undetectable to us using our unaided senses. However, if an object absorbs enough energy, for example if it is heated to a high enough temperature, it will emit photons of energy sufficient to be visible to the human eye.
(NOTE: The temperature at which different materials emit visible light differs.)

We can see this happening when the flow of electrical current through the filament of a light bulb is gradually increased.