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Optical Devices

by Ron Kurtus (revised 30 September 2012)

Have you ever wondered how a camera works? Or why you need a flashlight to see in the dark? Or how a magnifying glass can make things look bigger? Or why a rainbow looks so colorful?

There are many devices that use the visible electromagnetic waves we call light. Some of these devices create light, some detect light, and others manipulate the beams of light for some use.

This lesson will answer those questions. Useful tool: Units Conversion

Light passes through some materials

Light easily passes through air and glass. Such materials are called transparent.

There are some materials that only allow some of the light to pass through them. If only a percentage of the light can pass through, the material is called translucent.

Some materials only allow selected wavelengths to pass through. Colored glass may block some colors, while allowing only one or two specific wavelengths to pass through it.

It is obvious that it doesn't pass through materials like steel or wood.

The ray model of light

Light normally travels in a straight line. Some useful properties of light can be studied by considering light as a single ray or straight line: reflection, refraction and dispersion

Reflection

When incoming light reflects off an object, the incident ray has the same angle as the reflected ray, when measured from a perpendicular from the point of reflection. The two angles are equal, and this is called the Law of Reflection.

For example, when light hits a flat mirror, the angle of reflection is the same as the incoming angle.

Refraction

Light rays are bent, or refracted, at the boundary when passing from one transparent media to another. For example, you have seen how light is bent when it passes from water to air.

The amount of refraction depends on the incident angle and the index of refraction of the material. The index of refraction is defined as the ratio of the speed of light in a vacuum to the speed of light in the media.

In certain situations, when the refracted angle is 90 degrees to the incident angle total internal reflection takes place. This limit to the angle of incidence is called the critical angle, and all light rays with an incident angle at or beyond this angle are reflected internally.

Dispersion

When visible light passes through a glass prism, the light is broken into its colors. This occurs because the index of refraction is different for each color, with short wavelengths refracted more than larger ones. Thus the different colors are bent at gradually increasing angles, and the colors are spread apart.

This property of separating a beam of white light into a spectrum is called dispersion.

Prisms

Light passing through a glass wedge or prism will not only be bent by refraction, but will also be dispersed into its spectrum of colors. In studying light, Isaac Newton used a prism to discover that white light was composed of the spectrum of colors.

Spectroscope

The spectroscope is a device that is used to determine the elements present in a luminous object or gas. By examining the spectrum produced when the emitted light is passed through a prism you can determine the elements creating the light. Each element has its own characteristic location and color in the spectrum. With the spectroscope, light coming from our sun and other stars has been investigated to determine the elements present.

Lenses

The ray model of light is very useful in explaining lenses.

A lens is a piece of glass or any other transparent material with two curved surfaces, or with one curved and one flat surface. A convex lens is a lens that is thicker in the middle than at its edges. Refraction through such a lens causes parallel light rays to converge (meet) at a point called the principal focus. The lens of your eye is a double convex lens that focuses an image on the retina of the eye.

Lens in the eye

If a person is farsighted, the eyeball is too short from front to back, and the point of focus will fall behind the retina. Such people can see only distant objects clearly. Glasses with convex lenses are used to correct farsightedness.

Magnifying glass

A magnifying glass is a double convex lens that behaves like two prisms connected together at their bases. It can concentrate light rays from the sun to such a degree that they can bum a hole in paper or cause a fire.

Focal point of a convex lens

A convex lens has a focal point. Light coming from that point is refracted by the lens to become parallel or vice versa. An object that is beyond the focal point of a lens will be focused upside-down on the other side of the lens.

The camera and the eye

Both a camera and the eye have a convex lens which focuses the image upside-down on its detector. The camera uses film to detect the image, while the eye has its retina to detect the image.

Concave lens

A concave lens is thinner in the middle than at its edges and causes light rays to diverge (spread apart). If a person's eyeball is too long from front to back, the point of focus will be in front of the retina and the person will be nearsighted. Such people can see only close objects clearly. Concave lenses are used to correct near-sightedness.

Microscope and telescope

The ordinary light microscope is used to investigate tiny structures such as cells and bacteria that would be otherwise invisible to the unaided eye. The telescope also makes use of lenses to bring distant objects into view.

Special characteristics of light

A wave model of light can be used to explain diffraction, interference, and polarization, all of which provide strong evidence for the wavelike nature of light.

Diffraction

Diffraction is the bending of light around the edge of an object or spreading of light in an arc after passing through a tiny opening.

Did you ever see waves hit a breakwater and then sort of go around the obstacle at the edges? The same thing happens with light at edges.

By looking at the tiny grooves in a phonograph record, you can see another example of diffraction. Look at a sharp angle, and you should see how light is spread into various colors by the diffraction.

Interference

Since light is a wave motion, it is possible that if the waves are out of phase with each other, that they could cancel out each other. Another example is that is waves are of slightly different wavelength, they could cause "beat frequencies." This is similar to hold two tuning forks that are slightly different near each other. They produce a throbbing sound or slow beats.

Interference occurs when light passes through two small slits or holes and produces an interference pattern of bright lines and dark zones.

Oil slicks on a wet pavement are caused by the interference of the light passing through and reflecting off the ultra-thin oil film.

Polarized light

Polarized light is light that is allowed to vibrate in only one direction. This is similar to trying to vibrate a rope that passes through a picket fence: the rope can only vibrate is a direction parallel to the pickets.

Polarized light vibrates in one direction only, in a plane. Certain materials can polarize light, by means of reflection, or by scattering. Only the transverse wave model can explain polarization.

Polaroid sunglasses are used to cut down glare. They filter out light that is reflected off surfaces, but allow the other to pass through.

Summary

You have seen the interesting characteristics of light and now understand the fascinating things light can do.

You don't need glasses to have a good way of looking at things

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