Chapter 18: Refraction and Lenses

Section 18.1 Refraction of Light

Section 18.2 Convex and Concave Lenses

Section 18.3 Application of Lenses

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18 Table Of Contents

BIG IDEA Lenses refract light and create images.

Refraction and Lenses

• Refraction - when light crosses a boundary between two mediums, it bends.

• When light encounters a transparent or translucent medium, some light is reflected from the surface of the medium and some is transmitted through the medium. Amount of refraction is dependent on the medium

• What do you notice about the light rays after they cross the boundaries?

Light and Boundaries

• When you shine a narrow beam of light at the surface of a piece of glass, it bends as it crosses the boundary from air to glass.

• The bending of light, called refraction, was first studied by René Descartes and Willebrord Snell around the time of Kepler and Galileo.

Snell’s Law of RefractionRefraction of Light

• When light travels from air through glass and back to air, it refracts toward and then away from the normal.

Snell’s Law of Refraction (cont.)Refraction of Light

The ratio of the sines of the angles of incidence and refraction of a wave are constant when it passes between two given media.

Video: https://www.youtube.com/watch?v=k8oK67nlm3M

Index of Refraction (n) – determines the angle of refraction of light as it crosses the boundary between two mediums.

Snell’s Law of Refraction

The value given for air contains additional significant figures to distinguish it from that for a vacuum. Use the value for n of 1.00 in problems

The relationship found by Snell is also valid when light goes across a boundary between any two materials.

• When light travels from glass to air it moves from material having a higher index of refraction to one with a lower index. In this case, n1 > n2.

• To keep the two sides of the equation equal one must have sin θ1 < sin θ2.

• The light is bent away from the normal.

• Note that the direction of the ray when it leaves the glass is the same as it was before it struck the glass, but it is shifted from its original position.

SECTION18.1

Refraction of LightSnell’s Law of Refraction

Relationship to AstronomyBlood Moon

https://www.youtube.com/watch?v=oiAm_Ksg7fo

Total Internal reflection - An optical phenomenon that occurs when light strikes a boundary between two mediums at an angle of incidence that is greater than the critical angle and all light reflects back to the region of the higher index of refraction.

Critical angle – the angle of incidence in which a refracted light ray lies along between the two medium

https://www.youtube.com/watch?v=qGEit41xpfA

Critical thinking: Are the light waves travelling faster near the surface of the road or near the top of the truck? - index of refraction is smaller for warmer air. Due to the inverse relationship between the speed of light and index of refraction, the speed of light will be greater near the road surface.

Dispersion of light

Light from the sun is dispersed by water droplets by water droplets at various positions in the sky, a complete spectrum is visible.

Critical thinking

Will it be possible for a rainbow to be closed enough to touch?- A rainbow would not be seen if you are close enough to touch it. You can see a rainbow because the colors spread out with distance from the water droplets where they are dispersed.

Second-order rainbow

Light reflected within the raindrops can cause the appearance of a secondary rainbow. It is fainter and the order of colors are reversed.

A third-order rainbow is seen rarely since it is even fainter than the second-order rainbow.

Think criticallyWhy can you see the image of the sun just above the horizon when the sun itself has already set? - because of bending of light in the atmosphere (refraction)

In what direction would you have to be looking to see a rainbow on a rainy late afternoon? - in the east, because the sun sets in the west and sunlight must shine from behind you in order for you to see a rainbow.

Section 18.2

Convex and Concave Lenses

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18 Table Of Contents

A lens is a piece of transparent material, such as glass or plastic, that is used to focus light and form an image. Each of a lens’s two faces might be either curved or flat.

Equations

Defects of spherical lenses

Defects of spherical lensesChromatic Aberration - is an effect resulting from dispersion in which there is a failure of a lens to focus all colors to the same convergence point. 

MAIN IDEAPeople see objects that they could not otherwise see by using lenses.

Application of Lenses

Bellringer: Hold a pencil about 10cm from your eyes and focus it. Then, slowly look to a point at least a meter away and focus it. Do this several times and observe what will happen.

How our eyes see things

How to correct eye defects

http://kids.niehs.nih.gov/games/illusions/

lots_of_illusions.htm