Light

Light

By GeniusHuman

How Light travels

Sources of light

A light source is an object that emits its own light. Hot objects, such as flames and the sun are light sources. A torch bulb has a hot filament, so it is also a light source.

Some light sources are not hot - such as, a computer screen.​

A Torch bulb is a light source, for example.

The photograph of the sun behind the clouds shows us how light travels.

You can see the straight rays of light spreading out from the sun. This tells us that light travels in straight lines.​

Straight lines

An object which is not a source of light is described as a non-luminous object.

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Example : Chair​

Non-luminous object

An object which is a source of light is described as a luminous object.

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Example : Light bulb​

Luminous object

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When light strikes an object, different things can happen.​ It depends on the material of the object is made of.

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• ​The light may pass straight through the object. We say that it has been transmitted. The material is transparent.

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• The light may be absorbed by the object (The object get a little warmer). The material is opaque.

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• The light may bounce off the object. We say that it has been reflected.

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​Sometimes, two of these things can happen, or all three. For example, glass transmits light but it may also reflect some of it.​

A shadow forms when an opaque object blocks the light. It is formed because light travels in straight lines - it cannot bend round corners.

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​In the diagram, a pencil stands in front of a white card screen.

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When light from the torch is directed at the pencil, the shadow of the pencil appears on the screen.​

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Forming a shadow

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How reflections form

Mirrors reflect light. White paper reflects light. Why do we see a clear image on the mirror but not on the paper?

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A sheet of paper has a rough surface. When rays of light strike the paper, they are scattered in all directions.​

A mirror has a very flat, smooth surface. Rays of light bounce off a mirror without being scattered.​

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​A piece of paper scatters light in all directions.

A mirror reflects all the light in the same direction.

Predicting the path of light

If you shine a ray of light ​at a mirror, it reflects off the mirror.

The law of reflection tells us about the direction in which the ray s reflected. ​

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• The mirror is represented by a straight line.

• They ray of light coming in is called the incident ray, and the one going out is called the reflected ray.

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• To predict the direction of the ray, we need to draw the normal to the surface of the mirror. The normal is a straight line drawn at right angles (90°) to the mirror at the point where the ray is reflected.

Law of reflection

The law of reflection of light says that two angles marked in the diagram are equal. (Note that each of the angles is measured from the normal to the ray, not from the ray to the middle).

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Law of reflection :

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angle of incidence = angle of reflection​

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The photograph shows a pencil in a glass of water. The pencil looks as if it is broken at the point where it enters the water.

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How light bends

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Transparent materials

Water and glass are transparent materials. Light can pass through them, but something happens when light enters or leaves such a material.

The pencil in the picture appeared to be broken ​at the point where it entered the water.

the change of direction of a light ray, when it enters or leaves a transparent material is called refraction.​

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​The ray travels in a straight line to the surface of the water. This is where it bends.

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To understand where it bends, ​we draw the normal to the surface at the point where the ray bends.

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We can see that the ray bends away from the normal as it leave the water​.

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Rays changing direction

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The spectrum of white light

Splitting light

You​ can see the colors of the rainbow for yourself by sending a ray of white light into a glass prism. (A prism is a triangular glass block).

When the light enters the prism, it bends - it is refracted. It also bends as it leaves the prism.

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The white light is split into a spectrum of colors. They are the same colors that you see in a rainbow. The splitting up of white light into separate colors is called dispersion.

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The colors of the spectrum always appear in the same order:​

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​red orange yellow green blue indigo violet

Spectrum

Explaining dispersion

Because a spectrum appears when light passes through a glass prism, some people imagined that it was the glass that gave the color to the light. Isaac Newton realized this was incorrect. He showed white light (such as light coming from the sun) is a mixture of all the different colors of the spectrum.

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Dispersion happens because of refraction. When white light enters a block of glass, some colors bend more than others. Violet bends the most, red the least. This means that the ​different colors travel off in different directions, and so we can see them separately.

Colored light

To make colored light, a filter is placed in front of a bright white light.

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A filter is a piece of colored glass or plastic​. It only lets through some of the colors which make up white light. It absorbs the other colors.

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For example, a red filter lets through red light (and a bit of orange)​. It absorbs yellow, green, blue, indigo and violet

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Adding colored light

Put red, green and blue filters on three torches​. Shine them so that their different colors overlap. Where al three colors meet, you will see white light. This is because each filter lets through one-third of the spectrum. When you use them together, all color of the spectrum are present, and they add up to give white.

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Red, green and blue ​are called the primary colors of light. When two or more colors of light are mixed together, this is called color addition.

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Reflecting colored light

Grass is green. When you see grass, green light from the grass is entering your eyes.

Grass is green because it reflects green light from the sun. It absorbs all of the other colors of the sunlight. The grass has removed most of the colors of the sunlight. This is called color subtraction.

Other colors work the same way.

For example, a red object reflects red light and absorbs all other colors. White objects reflect all light, and black objects absorb all colors.

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Thank you

A quiz from this topic will be uploaded soon!