Real Images (Cambridge (CIE) IGCSE Physics): Revision Note
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Converging lens - real image
Constructing converging ray diagrams
The three main rules for constructing ray diagrams are as follows:
1. Rays passing through the principal axis will pass through the optical centre of the lens undeviated
2. Rays that are parallel to the principal axis will be refracted and pass through the focal point f
3. Rays passing through the focal point f will emerge parallel to the principal axis
Drawing converging ray diagrams of real images
Object placed beyond 2f
When an object is placed beyond 2f (to the left of the lens), the image that forms (to the right of the lens) will have the following properties:
The image forms... | between f and 2f |
The nature of the image is... | real |
The orientation of the image is... | inverted |
The size of the image is... | diminished |
Object placed at 2f
When an object is placed at 2f (to the left of the lens), the image that forms (to the right of the lens) will have the following properties:
The image forms... | at 2f |
The nature of the image is... | real |
The orientation of the image is... | inverted |
The size of the image is... | the same |
Object placed between f and 2f
When an object is placed between f and 2f (to the left of the lens), the image that forms (to the right of the lens) will have the following properties:
The image forms... | beyond 2f |
The nature of the image is... | real |
The orientation of the image is... | inverted |
The size of the image is... | magnified |
Worked Example
Draw a ray diagram to show how a converging lens can be used to form a diminished image of a real object.
Label the object, image and principal foci of the lens on your diagram.
Answer:
Step 1: Start by drawing and labelling a principal axis and the lens as a line or a very thin ellipse
Step 2: Mark and label the focal points on each side of the lens
Step 3: Draw and label the object at a distance greater than the focal length on the left side of the lens
Tip: For a diminished image the object should be placed a distance of at least 2F away from the lens
Step 4: Draw a ray through the optical centre of the lens
Step 5: Draw a second ray from the object to the lens which is parallel to the principal axis
Step 6: Draw the continuation of the ray passing through the focal point on the right side of the lens
Step 7: Draw and label the image at the point where the rays meet
Step 8: Check your final image and make sure everything is included to gain the marks
For a three-mark question, examiners will be looking for:
One ray drawn through the optical centre of the lens
A second ray drawn which produces a diminished (smaller) image (which must pass through a labelled focal point)
Both the object and the image must be drawn and labelled correctly
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