GCSE Physics How to construct convex lens ray diagram when object O is a distance beyond 2F (2 x focal length) from centre of lens image is real inverted smaller than object IGCSE/GCSE Physics revision notes

GCSE level Physics exam revision notes on OPTICS

Optics: Lenses: Part 6. Constructing the convex lens ray diagram when object O is at a distance beyond 2F (2 x focal length) from the centre of the lens

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INDEX of notes on optics: lens types, properties, correcting eye defects

6. Convex lens ray diagram for when object is at a distance beyond 2F (2 x focal length) from the lens

Ray diagram 6a When the object O is beyond a distance of 2F from the convex lens
- 6a converging lens
- If the object is a long way from the lens the image is formed between F and 2F and is standing on the axis.
  - Apart from the axis line, this is essentially a 2 ray diagram for an object 'standing' on the axis line.
- (i) Draw a ray from the arrow tip parallel to the principal axis into the lens.
  - Since this is parallel to the axis, beyond the lens, the ray must continue down through the principal focus (in this case beyond an F distance to the right of the lens). Check this line in ray diagram 6a above.
- (ii) You then draw a line, again from the top of the object, down through the centre of the lens, and continue the line until it is beyond intersecting with the first ray you drew.
- The intersection point gives you the position of the bottom of the image and the inverted arrow gives you the size of image I. From this you can see that ....
- The image I is real, inverted (upside down!) and smaller than the object O.
  - If the object O is at infinity, the focussed image is at a distance F beyond the lens.
  - This means the further the object O is from the lens, the nearer the image I is to distance F.
  - This is also the image formed in a telescope from a very distant object like a star which is so far away that the incoming rays are effectively parallel.
  - The image can then be magnified by another lens or lenses in conjunction with an eyepiece or camera.
- 6b convex lens
- Above is quick sketch 6b of how to do the ray diagram 6a on graph paper. If done very carefully to scale, you can then calculate the height of the image I and the distance from the lens to the image I.
Below is a more elaborate graph paper ray diagram 6c for a convex lens where the object is placed at a distance beyond 2F from the lens, BUT, above the central axis of the lens - four rays are marked (i) to (iv), each intersecting pairs of lines ('rays') gives you the top and the bottom of the image.

constructing ray diagram convex lens object beyond 2F inverted real image smaller size than object gcse physics igcse 6c

(i) Draw a line from the top of the object to the lens and, after the lens, down through the focal point F.
(ii) Draw a diagonal line from the top of the object down through the centre of the lens and beyond the intersection with ray (i).
- The intersection of rays (i) and (ii) gives you the position of the bottom of the inverted image.
(iii) Draw a line from the bottom of the object parallel to the principal axis and, after the lens, diagonally down through F.
(iv) Draw a line from the bottom of the object diagonally down through the centre of the lens and beyond the intersection with ray
- The intersection of rays (iii) and (iv) gives you the position of the top of the inverted image.

If the object is a F, the image is at infinity, which is not very useful? (not needed for GCSE/GCSE physics?).
- 6d convex lens
- Graph paper ray diagram 6d to show how an image is formed at infinity.

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Keywords, phrases and learning objectives for properties and uses of lenses

Know how to construct the convex lens ray diagram when the object O is at a distance beyond 2F (2 x focal length) from the centre of the lens.

Know that the image is real, inverted and smaller than the object.

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