Fundamentals of geometric optics Topics 7 & 8

What is the positive and negative representation of thin lenses in ray diagrams?

Where are F and F’ on positive and negative lenses in ray diagrams?

• Positive: F is left and F’ is left

• Negative: F is right and F’ is left

If the optical media on the it her side of the lens are the same (eg a lens in air) what does this mean about the focal points?

• Equidistant from the lens

• f and f’ have the same value but different signs

Why is the minimum number of rays needed in a ray diagram two?

• We are assuming paraxial imaging,

• therefore if two rays that originate at an object point meet at some image point, then any ray originating from that object point will also meet at the same image point (ideal imaging).

What are the 3 construction rays?

• Object ray: ray in object space, parallel to the optical axis, will refract to give an image ray (ray in image space) that goes through second focal points F’.

• Image ray: An object ray that goes through the first focal point will refract so the image ray is parallel to the optical axis

• Undeviated ray: for a thin lens, ray that goes through centre of the lens , doesn’t bend

Show how these rays are different in positive and negative lenses

Positive on previous flash card

Draw a simple ray diagram

Do all the object rays originate from different points?

No they all originate from the same point

Show how a real object can be imaged by a negative thin lens

Virtual, minified, erect

Draw what happens when a real object is placed between the first focal point F and a positive thin lens

Virtual, erect and magnified

Draw the ray diagram if an object at infinity

For objects infinitely far away, the image should always be at F’

What spaces do F and F’ belong to respectively?

• F for object space and only object rays going through F act specially

• F’ for image space

What should be remembered when drawing focal points?

• If f = -20cm and f’ = +40cm then F’ should be twice as far from lens than F on oppsodte sides

• Drawfocal points in relation to eachother

Draw and label an optical system consisting of 2 thin lenses

How can we work out the location, size and type of image for the first lens?

How can we work out the location, size and type of the image for the second lens?

Draw the diagrams for lens 1 and 2 in this problem