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Background Information for Looking at Light

Background Information for Looking at Light

Below you will find some commonly asked questions about light and lenses.

How important is our sense of sight?

We have five main senses: sight, hearing, taste, touch and smell. About 75% of the information humans receive about our environment comes from our sight, making it the most important of the five senses.

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What do we need in order to see?

We need LIGHT to see.

We need EYES to see, with optic sensors called rods and cones

To make sense of what we see, we need a BRAIN.

The optic nerve at the back of the eye connects to the central nervous system in the brain. The brain receives electrical impulses from our eyes which are interpreted as sight, but the brain adds two extra ingredients to the eyes' image; memory and interpretation. A newborn baby cannot identify objects in his/her environment but with time, accumulates information and learns to recognize familiar objects and people. The brain can be thought of as a giant filing cabinet which collects and files the information we receive from our eyes and stores it all in the memory. When we see a familiar object, such as a chair, we use our eyes, memory and imagination to interpret information relating to its structure and function.

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What is light?

the sunLight is a form of radiant energy. We only see a very small portion of what is called the electromagnetic spectrum. If you hold a magnifying glass in the sunlight and focus it on a piece of paper, you can burn a hole in the paper. This heat energy does not come from the magnifying glass. It comes from the Sun, and includes not only visible light but also ultra violet and infra-red energy.




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What are common sources of light?

FlashlightWhich of the following are sources of light?

Sun, stars, fire, candle, Moon, lightbulb ,flashlight, mirror candle

You are right if you said the sources are Sun, stars, fire, candle, lightbulb, and flashlight. The Moon and mirrors only reflect light. Some of the sources are natural, and some, like the lightbulb and the flashlight, are artificial, creating light with electricity.

Did you know it takes the light from the Sun approximately eight minutes to reach the Earth? The reflected light from the Moon takes about one and a quarter seconds to reach the Earth. Light travels at just under 300,000 kilometers per second.


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How do we see?

We see by direct and by reflected light. If you turn off the light in a windowless room, you will not be able to see. We see an object because the light from the source bounces off the object and is reflected into our eyes.

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How does light bounce?

The angle at which a light ray hits a polished reflecting surface determines the angle at which it will leave. All surfaces reflect to some extent.

The angle of incidence (incoming ray) equals the angle of reflection (reflected ray).

Try this out in a dark room with a flashlight and a mirror.

You have probably heard of fibre optics. These are fine transparent rods of plastic or glass that can carry telephone messages and other information. In fibre optic applications, the message is transformed into a light ray, which is then shot through one end of a transparent rod of plastic or glass, coming out the other end. This can be done even if the rod is bent into a curve. The transparent rod is made of two layers; the light travels in the inner layer and is reflected internally by the second, outer layer. Very little light is lost through the sides of the rod because there is a high degree of total internal reflection. Nearly 100% of the beam is reflected. That is how the light travels down the tube. At the end of the tube, the light is transformed again back into the original voice message or information.

Light ray travelling along an optical fibre

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How does a periscope work?

periscopePeriscopes use mirrors (or 45 degree glass prisms) to direct light around corners. In a simple periscope, 2 flat mirrors each reflect the light (usually through 90 degrees) enabling an observer to see over walls, around corners and over people's heads in a crowd. Periscopes are particularly important in submarines because they enable the crew to view objects on and above the surface of the water.





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Can you "bend" light?

Light always travels in straight lines, but we can use convex or concave lenses to change its direction. A lens is a piece of glass or plastic that has been ground and polished to make it curve in a certain way. Light changes its direction just as it enters another substance or medium with a different density. Although the light rays alter direction, they still travel in straight lines. This bending of light is called refraction. Even light passing from the Sun or a star through the atmosphere is bent or refracted, especially when the object is low on the horizon.

Light travels at just under 300,000 kilometers per second in air but slows down slightly when it enters a more dense medium such as glass. When it leaves the glass, it speeds up again. The steeper the angle at which a light ray enters or leaves the new medium, the more it is bent or refracted.

The point at which the rays come together , the focal point, (real or imaginary), will produce an image of the object emitting or reflecting the light rays.

Bending light with lenses is the basis for eyeglasses, binoculars, refracting telescopes and microscopes. Combinations of lenses can be used to make things look bigger, as in a microscope or to make things look closer, as in a telescope.

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The microscope

Microscopes have become very important instruments, especially in the fields of biology and medicine. Modern microscopes use several convex lenses to make nearby objects appear larger. These objective lenses and eyepieces are made with several lens elements, each with different densities to control the sharpness and colour to give the best possible image.

Some optical microscopes can magnify objects hundreds of times.

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The telescope

Reflecting telescopes have become very important in the exploration of space. Telescopes use both mirrors and lenses to produce close-up images of distant objects such as stars and planets. In reflecting telescopes, the first or primary mirror is concave and is the largest mirror. The larger it can be, the better, because as the size goes up, the amount of light collected goes up.

Reflecting telescope

A telescope is really a big "light bucket". The primary mirror reflects the incoming light to a focal point. From this "prime focus", another mirror (flat or curved) reflects the light to a focus at a more convenient point for you to place your eye or attach a piece of instrumentation, such as a camera - usually at the back or side of the telescope tube.

In refracting telescopes, large lenses bend or refract the light to a focus, but these telescopes are much more difficult and expensive to build and can only be about 1 meter in diameter. By contrast, the largest reflecting telescopes are now 10 meters in diameter.

The museum's own telescope

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The spectrum

Light is the only source of colour in our world. White light is a mixture of all colours. When white light travels through a prism, it is split into the spectrum, the seven observable colours of the rainbow: red, orange, yellow, green, blue, indigo, violet. Each colour has its own wavelength and energy. At one end of the spectrum, violet has the shortest wavelength (most energy), and at the other end, red has the longest wavelength (least energy).

When white light passes through the glass of a prism, the prism bends the different wavelengths of light, each to a different degree. Violet bends the most, and red the least, with the other colours falling in between. The light emerges from the prism broken into the spectrum, all the colours of the rainbow side by side. We only see a very small portion of the electromagnetic spectrum. Flies can see in the ultra violet range - beyond violet. With electronic devices, we can now detect most parts of the electromagnetic spectrum.

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Why is a red apple red?

appleWhen white light shines on a red apple, the apple skin absorbs all of the colours in white light except red. The red light is reflected into our eyes and we see the apple as red. We see a white object as white because it reflects all the colours into our eyes. An object that appears black has absorbed all the colours of the white light and reflects nothing back to our eyes.

If you place a black shirt and a white shirt outside in the sunlight, which one will heat up more quickly? The black one will because it absorbs all the white light coming from the Sun, and since light is energy, the shirt will heat up.

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What is infra-red light?

Infra-red light has longer wave lengths than red light, and though we can't see it, we can feel it as heat radiation. The heat we feel from the Sun comes mostly from infra-red radiation. Older remote controls for TV sets used infra-red radiation. Infra-red radiation behaves just like light. If you have an older remote, try bouncing the remote control signal for your TV off a mirror to turn on the set. Did it work?

(How do you know if you have an infra-red remote control? If it only works when you point it straight at the TV, CD player or VCR, then it's probably an infra-red device.)

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What is ultraviolet light?

At the other end of the spectrum from infra-red light can be found ultraviolet light, with a wave length just a little shorter than violet light. It comes to us from the Sun in the form of invisible radiation. Weather reports now give warnings if the ultraviolet radiation will be strong. Always wear sunscreen to protect your skin if you will be outside.

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What are optical illusions?

Sometimes the brain is deceived by information received from the eyes. Optical illusions are caused when differences occur between our perceptions or expectations and the image seen by the eye. Perception is a combination of seeing with memory and imagination. It means understanding the meaning of images seen by the eye and sent to the brain. For example, if you saw a train speeding towards you but had not learnt to move out of the way, you would almost certainly be run over!

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