Physics 10 Reflection and Refraction

Law of Reflection

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Plane Mirrors

Law of Reflection ⇒ The angle of reflection equals the angle of incidence.

Plane Mirrors

⇒ The reflected light from an object looks as

⇒ The image appears to be as far behind a

through it came from a point behind the mirror. This is where the image is located.

plane mirror as the object is in front of the mirror.

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Plane Mirrors

Exercise 14 Hold a pocket mirror at almost arm’s length from your face and note the amount of your face you can see. To see more of your face, should you hold the mirror closer or farther, or would you have to have a larger mirror? (Try it and see!)

⇒ The image from a plane mirror is upright, the same size as the object, and as far behind the mirror as the object is in front of it.

Plane Mirrors

⇒ Note that in your (flat) pocket mirror that the amount of your face you can see is twice the size of the mirror – whether you hold it close or at arm’s length!

Specular Reflection

⇒ To view one’s full length in a mirror, only a half-length mirror is needed. ⇒ Reflection from a smooth surface is called specular reflection.

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Diffuse Reflection

⇒ Reflection from a rough surface is called diffuse reflection.

Refraction of Light ⇒ To understand why light bends when passing from one transparent material to another, imagine a cartwheel rolling from a sidewalk onto grass. ⇒ Because the wheels roll slower in the grass, the left wheel will slow down first causing the cartwheel to change directions.

Refraction of Light

Refraction: The bending of light when it passes from one transparent material to another.

Refraction of Light ⇒ Similarly, when light hits a transparent material like water where the speed of light is slower, one side of the wavefront slows down before the other side.

⇒ Because light travels slower in the water, the left side of the wavefront will slow down first, causing the light to bend.

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Exercise 20

Apparent Depth

A pair of toy cartwheels are rolled obliquely from a smooth surface onto two plots of grass, a rectangular plot and a triangular plot as shown. The ground is on a slight incline so that after slowing down in the grass, the wheels will speed up again when emerging on the smooth surface. Finish each sketch by showing the path of the cartwheels inside the grass and on the other sides.

⇒ Because of refraction, the apparent depth of objects in water is less than their actual depth. (Objects are deeper than they appear.)

Apparent Depth

Exercise 23

Place a glass test tube in water and you can see the tube. Place it in clear soybean oil, and you may not be able to see it. What does this tell you about the speed of light in the oil and in the glass?

⇒ Because of refraction, the apparent depth of the end of the ruler inside the water is less than the actual depth. This is why an object in water appears to bend.

⇒ The speeds in both glass and soybean oil are the same, so there is no reflection or refraction between the glass and oil.

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Dispersion

Dispersion

⇒ The speed of light in a transparent material depends upon the frequency of the light (and the type of material).

⇒ Because different frequencies of light travel at different speeds in a transparent material, the frequencies are bent (refracted) by different amounts.

Dispersion: The separation of light into different colors arranged according to their frequency.

Rainbows

Rainbows

⇒ Dispersion occurs when sunlight hits a water droplet. This is what causes rainbows. ⇒ The incoming sunlight enters the water droplet and different frequencies are bent (refracted) by different amounts. ⇒ When this refracted light hits the back of the water droplet, some of it is transmitted back into the air (not shown) and some of it is reflected back into the water droplet. ⇒ The light reflected from the back of the water droplet then gets

⇒ The different colors of a rainbow are the result of the dispersion of sunlight by millions of water droplets that act like tiny prisms.

bent again (refracted) as it leaves the water droplet.

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Rainbows

Rainbows

⇒ Although each water drop disperses a full spectrum of colors, a person can only see one color of light from each drop.

⇒ Because of this, two people in different positions do not see the “same” rainbow.

Rainbows

Rainbows

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Rainbows

Rainbows

⇒ Sunlight can sometimes be reflected twice within a single water droplet.

Single reflection – single rainbow.

Double reflection – double rainbow.

Green Flash

Green Flash

⇒ A simplified explanation of green flashes is that our atmosphere acts like an upside down prism. White light is dispersed with blue on top, green near the top, and red on the bottom.

⇒ Sometimes a momentary flash (lasting 1 or 2 seconds) of green light can be seen when the sun sets.

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Green Flash

Blue Flash

⇒ At the moment of setting, the red is cut off by the Earth, the blue is removed because of scattering, and the green survives to give the green flash.

⇒ Under ideal atmospheric conditions, it is possible to see a blue flash.

Total Internal Reflection

Total Internal Reflection

⇒ When light tries to go from one transparent material into another transparent material where the speed of light is greater, the light will get totally internally reflected (no light gets refracted out) if the angle the light hits at is greater than a certain angle called the critical angle.

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Total Internal Reflection

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