Light Refraction and Lenses

8/13/2019 Light Refraction and Lenses

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LIGHT


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Refraction

Refraction Fact #1: As light goes from one

medium to another, the velocity CHANGES!

Refraction Fact #2: As light goes from one

medium to another, the path CHANGES!


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RefractionGoing from Air to Water

If a light ray goes from a LOW refractive Index(FAST MEDIUM) to a HIGH refractive index

(SLOW MEDIUM), its speed DECREASES and

the angle BENDS TOWARDS the normal


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RefractionGoing from Water into Air

If you are going from a HIGH refractive indexto a LOW refractive index, your speed

INCREASES and the angle BENDS AWAY the

normal


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There is one exception

That is when light hits head-on,

perpendicular to the boundary. The light does

not bend but its speed still changes.


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Total internal reflection

happens when the angle between the light rayand the normal is greater than the criticalangle.

The critical angle is formed when the light raytravels along the boundary between the twosubstances.

Optical fibres use multiple total

internal reflections to transmit light.


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Copy the diagrams into your workbook.

Draw a normal wherever the light rays enter a

new substance.

Demonstrate what will happen to the rays as

they enter and exit from the substances by

continuing the ray through the shape and out

the other side.


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Apparent Depth Prac

Coin and Fish


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Why does it look bent away from

normal and not towards??

Light from the lower part of the ruler is travelling

into a region of lower optical density (air), and so

has been bent away from the normal.

The image in the water is actually an illusion

an

image our brain constructs based on where light

from the lower part of the ruler appears to come

from. It assumes that the light travelled in a perfectly

straight line, even though it didnt.


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Part B: Refraction in water - ANSWERS

1 a Refract; towards.

b Emerge; away from. c Incident; a glass surface; no.

2 When light rays at an angle to the surface

travel from a medium of low density to one ofhigher density, the light rays refract

towards the normal.

3 a From the tip, to the eye.

4 a From the coin to the eye.


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Lenses

There are two main types of lenses:

convex lensesthese curve outwards and are

fatter in the middle

concave lenses

these curve inwards (a little

like a cave) and are thinner in the middle.

Convex lenses Concave Lenses


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LensesAn application of refraction

There are 2 basic types of lenses

A converging lens (Convex)

takes light rays and bring

them to a point.

A diverging lens (concave) takes

light rays and spreads them

outward.


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Movement of Light through Lenses

CONVEX LENS

In a convex lens, an incoming ray parallel to theprincipal axis is refracted through the principal

focus (F).


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Concave Lens

In a concave lens, an incoming ray parallel to

the principal axis is refracted so that it

appears to come from the principal focus (F).


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Movement of Light through a Lens

The distance from the centre line (plane) of thelens to the principal focus is called the focal

length of the lens.

A ray passing through the centre of either type

of lens is unaffected.

As with all images, rays of light that come from

a part of the object come together again at

that same part of the image.


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Focal Length

The greater the curvature of a lens, the moreit bends light and hence the shorter the focal

length.


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Image type and Location

Convex lenses produce two different types ofimages, depending on where the object is

located.

If the object is at a distance greaterthan thefocal length of the lens, a real image is

formed.

A real image can be projected onto a screen


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Convex Lenses

If the object is at a distance less than the focallength of the lens, a virtual image is formed.

This image cant be projected onto a screen.


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Concave Lenses

Concave lenses produce only virtualimages.


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Finding the focal length

Rays coming into a lens from a distant object arealmost parallel and form an image very close to

the focus.

We can then measure the distance from lens toimage to determine the focal length of the lens.


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Images in a Convex and Concave Lens Prac

AIM: To investigate the image formed by different convex

lens and concave lens Complete Convex pracusing 2 different convex lenses


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Questions CONVEX

1) What happened as the object was brought closer to the lens?

2) When does;

a a real image (on a screen) is obtained

b a virtual image (one that cannot be caught on a

screen) is obtained c no image is obtained

CONCAVE -

1) Assess whether it is possible to form a real image (one that maybe caught on a screen) using a concave lens.

2) Explain how the image changes as the object-to-lens distance isvaried.


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Answers


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Eye ball prac


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Light Refraction and Lenses