How to draw Ray Diagrams for Convex and Concave lenses

Concave step 1

Draw concave symbol with outwards pointed v-shapes at either end(because lens get wider towards its ends) → usually given in exam

Concave step 2

Add axis (horizontal line through middle of lens) + focal points on either side + add another dot on either side that is twice as far away from lens as focal points → usually given in exam

Concave step 3

Pick a point near top of object + draw incident rays coming out of the object at that point (one parallel to axis & one towards the centre of lens)

Concave step 4

1) the ray parallel to the axis will be refracted + to find the angle of refraction you draw a virtual ray (dashed line) between the point the ray hits the lens & focal point → draw a solid line as it continues past the lens

2) the ray going through the centre won’t be refracted at all + continue on straight

Concave step 5

Find the point where the second light ray and the virtual ray cross → that is the top of the image + repeat process to find bottom of the image (however, if the bottom of the object is on the axis, the bottom of the image will also be on the axis)

Convex step 1

Draw convex symbol with inwards pointed v-shapes at either end(because lens gets narrower towards its ends) → usually given in exam

Convex step 2

Add axis (horizontal line through middle of lens) + focal points on either side + add another dot on either side that is twice as far away from lens as focal points → usually given in exam

Convex step 3

Pick a point near top of object + draw incident rays coming out of the object at that point (one parallel to axis & one towards the centre of lens)

Convex step 4

1) the parallel to axis ray will be refracted so that it passed through the focal point of the other side of the lens + carries on going

2) the ray going through the centre won’t be refracted at all + continue on straight

Convex step 5

Find the point where both light rays meet → that is the top of the image

To comment on convex lens images

• If object is after the 2F → real + inverted + smaller

• If object is on the 2F → real + inverted + same size

• If object is between F and 2F → real + inverted + magnified

• if object is on F/between focal point(F) and lens → refracted rays won’t ever meet → you have to trace them backwards using virtual rays → find the point where they meet and draw image → virtual + upright + magnified

To comment on concave lens images

• Image produced always virtual

• Upright

• Smaller than the object

• On the same side of the lens as the object no matter where the object is