Physics P3 - Lenses and mirrors etc

basically information about the title, the most confusing part in the p3 syllabus, hope these are useful!

They are only basic, and easy ways to remember what's what.

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Basic info to start off with:

Magnification = Image Height
                        Object height

Angle of Incidence (i) = Angle of Reflection (r)

Refraction = When light rays change direction as they enter a different medium. If the speed of the ray is reduced, refraction is towards the normal line. If the speed is increased it is away from the normal.

Diverging = Spreading out.

Converging = Coming together

Virtual Image = When rays are diverging or when you draw it and the image formed is along dotted lines.

Real Image = Converging rays to form an image on a screen like your retina.

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When describing and image you need to say three things:

1) How big it is in comparison to the object

2) whether its upright of inverted

3) whether it's real or virtual.

Focal point = labeled F, halfway between the centre of the mirror and the centre of curvature (centre of the circle the mirror would make if it closed to make a circle)

Light that goes in parallel to the normal, must reflect through the focal point.

Light that goes in through the focal point, must come out parallel.

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Concave MIRROR

Concave mirror = ---> ) that shape.

The easiest way to remember it is going into a cave.

Forms converging rays.

If the object is between the mirror and the focal point then the image formed will be virtual.

If it is beyond the focal point it will be real.

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Plane Mirror

Plane Mirror = Flat mirror (like the ones you use at home).

The image is:

1) The same size as the object

2) It is as far behind the mirror as the object is in front

3) It is formed from diverging rays - meaning it is a virtual image.

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looks like this--->)

The light rays diverging rays.

Convex mirrors always form VIRTUAL images

conVex - Virtual.

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Concave LENSES

Concave lenses are thin in the middle and thick on the top and bottom, sort of like this shape       )(

The light rays diverge. They move towards the thick part of the lenses.

A line that goes through the centre of the lense just keeps on going, it does not change direction.

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Convex LENSE

convex lenses start thin and get big towards the middle. Sort of like this shape ()

The light rays converge, they move towards the thickest part of the lense. The centre.

A Line that goes through the centre of the lense just keeps on going, it does not change direction.

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very good, but some spelling mistakes make it a little hard to understand (sorry to be pernickety but thing/thin an/and etc. Other than that info is good, and this has helped me understand the focal point thing which i never got so THANK YOU :)

Jessica witt


oh sorry! i think i've got most of them now, haha i made this when i was REALLY tired trying to cram some extra revision in before bed! glad it helped!



Thanks so much! Really helpful :)

lozza day


they're ****

Jessica witt


i hate you.

Former Member


these are really useful, especially making the connection between conVex and Virtual :) however, you've used the same bracket - ) for both concave and convex mirrors? it's a little confusing

but otherwise it's been really useful :D 

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