Key Concept 1: Light travels in straight lines until reflected or refracted by
another object.
Visible light energy, which the human eye is able to detect, referred to in this
concept, represents only a narrow sliver of the electromagnetic spectrum.
This light is often produced when something becomes very hot. The Sun is our
biggest and most important source of light. With a surface temperature of about
5500°C, the Sun produces vast amounts of visible and invisible light. An electric
light bulb gives off light when electricity heats up a filament to about 2200°C,
causing it to glow.
Light rays from any light source travel outward in all directions along a straight
path–until they hit something. Then, light rays
can reflect (bounce), refract (bend), be absorbed, or can pass straight
through the material. Even though light can be absorbed by opaque objects and
transmitted through clear objects, only reflection and refraction of light energy
are the focus in the fifth grade.
Key Concept 2: Light is reflected when it bounces off of mirrors or shiny
surfaces.
The image we see in a reflection is a flipped image from the original. Because
light travels in straight lines, the angle of incoming light equals the angle of the
reflected outgoing light on shiny surfaces. On dull surfaces, light rays reflect in
many directions, called “scattered” light. The color the eye detects depends on
which wavelengths of light are absorbed and reflected. If dull objects scatter all
wavelengths, they appear white. If they absorb all wavelengths, they appear black.
If dull objects absorb all wavelengths but reflect red, the eye detects red objects.
When a surface is very smooth, like polished metal, glass, or the surface of still
water, light
is reflected like a
mirror.
Interesting results occur when the surface of a shiny object is curved. If the
reflective surface is convex (curves away from the incoming light), the light rays
bounce off and spread out (diverge), like looking at the backside of a spoon. The
image in a convex mirror appears to be smaller, upright, and covers a wider field
of view. Security mirrors on the sides of halls or building walls, and side mirrors
on trucks and cars, take advantage of the wide field of view created from a
convex mirror reflection. If the reflective surface is concave (curves toward the
incoming light), the light rays bounce off and come to a point (converge), like
looking at the bowl of a spoon. The image is reversed but larger. Telescopes, a
dentist’s mirror tool, and cosmetic mirrors take advantage of the magnified image
created from a concave mirror reflection.
Key Concept 3: Light is refracted, or bends, when passing from one medium
to another, such as from air into water.
A refracted image is distorted or a changed from the original. When light rays are
transmitted or pass from one medium to another (from air to water) other than
straight on, the speed of light changes slightly, which causes the light rays to
change direction slightly at the boundary of those two mediums. This bending of
light rays is called refraction. This happens when you look at a pencil in water,
and the part below the surface of the water looks bent. This refraction of light can
cause partially submerged objects in water to appear distorted.
Refracting light is important to the eye. The amazing flexible convex lens inside
the eye focuses incoming light that our brain interprets as the world humans see.
For those with poor vision, eyeglass lenses can refract or bend light rays to focus
better or to magnify the image. Convex lenses (curved away from incoming light)
are also used to make objects appear closer. Hand lenses, microscopes,
telescopes, or binoculars are examples of convex lenses. Concave lenses (curved
toward incoming light) are used to spread out images. Concave lenses are used in
glasses to correct nearsightedness (myopia), and in door viewers, or peepholes.
Remind students that light rays can also pass straight through to the other side
without refracting or bending the light, such as light passing through glass
windows.