Week 7 - mirrors - geometric and visual optics

What is a prism?

• two refracting surfaces inclined together at an apical angle (a)

How do you calculate the total angle of deviation?

• d = d1 + d2

• .d = i1 + i’2 - a

How does light travel through prisms in terms of Vergence?

• deviates light

• Doesn’t change Vergence

• Light is always deviated towards the base of prism

Steps to calculate deviation of a prism

1. Find i1’ (nsini = n’sini’)

2. Find i2 (feta = 90 - i’1)

3. Find y (all angles in triangle = 180)

4. Find i2 (90-y)

5. Find i’2 (sine law)

6. Calculate deviation using:

   • d = (i1-i’1) + (i2 - i’2) (d=d1+d2)

   • .d = i1 + i’2 - a

How do you calculate the minimum deviation?

• n = sin(a+dmin/2)/sin(a/2)

What does a porro prism do?

• inverts image

• Binoculars

What does a dove prism do?

• inverts image

• Can rotate image

When is a prism considered thin?

Apical angle is less than 10 degrees

• assume that i1 = 0

• Prism stands in air- na = 1

What is the unit for thin prisms?

• prism dioptres

• 1 prism diopter = tangent of 0.01

• 1 prism diopter causes a displacement of 1 unit at a distance of 100 units

How can the deviation (feta) in prism dioptres be calculated?

0(prism dioptres) = 100 x tan (feta in degrees)

To get tan feta do displacement/distance

How can the deviation be calculated in degrees in a prism?

• d = (n-1) x a

How can the prism power P (in prism dioptres) be calculated?

• p (prism dioptres) = 100 x tan (d in degrees)

How does the eye perceive the object through a prism?

• light is deviated towards the base of the prism

• Eye perceives the object to shift towards the apex

What are spherical mirrors used for?

• to produce changes in Vergence by reflection

What happens with concave mirrors and draw a diagram?

• reflected light converges

What happens with convex mirrors?

• reflected light diverges

What are the mirror equations?

• L’ = -n’l’

• L = n/l

• F = -2n/r

Draw a ray diagram for a concave mirrors

Draw a ray diagram for a convex mirror

Draw a diagram of an object at infinity of a concave mirrors

Draw a diagram of an object at infinity of a convex mirrors

Image at infinity concave diagram

Image at infinity convex diagram

How do you calculate mirrors single principal focal point?

• f = r/2

What is keratometry?

• measurement of radius of curvature of cornea

• Uses a keratometer

• Used fr fitting rigid contacts

• Applies spherical mirror theorey to cornea

How is the spherical mirrors theory applied to keratometry

• cornea acts as convex mirror

• Mires (illuminated object) imaged by cornea

• Size of image measured

• Allows for measurement of corneal radius of curvature

What do prisms fix?

• misalignment in the eye