P3aL7 Refraction and Lenses
Key Words
Concave - lens that bends inwards to the middle.
Converging - light rays coming together.
Convex - lens that bulges outwards.
Diverging - light rays going apart.
Focal length - distance between the lens axis and the focal point
Magnification - making something bigger.
Normal - a line at 90o to the surface.
Principal axis - the line passing through the centre of the lens.
Prism - a triangular block of glass that refracts light.
Ray diagram - a geometrical construction to help to decide what happens to the light rays.
Refraction - bending of a ray of light.
Refractive index - ratio of speed of light in air to the speed of light in a material.
Speed of light - 300 million m/s
Grade E
When light hits an air-glass boundary, there are three things that happen to it:
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Some light is reflected;
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Some light is absorbed;
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Most of the light is transmitted.
If we shine a ray of light at an angle, we find something a little strange. The ray does not carry on in a straight line as you might expect. Instead it bends inwards. This is called refraction.
Note the following:
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All angles are measured from the normal.
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The angle of incidence is greater than the angle of refraction. The ray therefore bends towards the normal.
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When the ray emerges from the glass, it bends away from the normal. The angle of refraction in this case is bigger than the angle of incidence.
Refraction occurs because the speed of light in air is greater than the speed of light in glass. The refractive index is given by the formula:
refractive index = speed of light in a vacuum
speed of light in material
The speed of light in air is very close to the speed of light in a vacuum. Refractive indices are always greater than on for a ray of light passing from air to a material. Refractive index has no unit.
For a prism, the ray diagram is like this, using a ray of monochromatic (single colour) red light.
If we use a ray of white light, we see that the light ray gets split into the colours of the rainbow (a spectrum).
Grade C
Lenses are optical devices that bend light by refraction:
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In a converging (convex) lens, the light rays come together.
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In a diverging (concave) lens, the light rays spread apart.
Both types of lens have uses.
Converging Lens
The converging lens brings parallel rays of light onto a single point like this.
Note the following:
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There are two principal foci (plural of focus). This is because the rays can come in either side of the lens.
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The image is real.
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The focal length is the distance between the focus and the lens axis. This is given the code F. So 2F means twice the focal length
Diverging Lens
The diverging (concave) lens makes the rays split apart or diverge, as shown in the picture.
Note how the diverging rays are extended back, and come together at the principal focus. The image of a diverging lens is virtual.
Ray Diagrams in Converging Lenses
We can determine where an image lies in relation to the objects by using a ray diagram. We can do this by using two simple rules:
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Draw a ray from the top of the image parallel to the principal axis. This ray bends at the lens axis and goes through the principal focus.
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Draw a ray from the top of the lens through the centre of the lens.
Where the two rays meet, that is where the image is found. The diagrams shows how we do a ray diagram step-by-step:
Step 1 Draw the ray parallel to the principal axis.
Step 2 Draw the refracted ray so that it passes through the principal focus.
Step 3 Draw a ray from the top of the object through the middle of the lens. This ray is undeviated.
Step 4 Where the rays meet, that is where the image is.
It is a good idea to draw your ray diagrams on graph paper as the following ray diagrams are. Be careful with your drawing; a small change in the angle of the undeviated ray can lead to quite a big change in the final position of the image. And PLEASE... Be a good chap and use a sharp pencil.
Grade A
There are no notes at this level.