Optics and Lenses: Images with Mirrors and Lenses, Refraction, and Optical Instruments, Slides of Physics

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31. Images & Optical Instruments

Images with Mirrors

Images with Lens

Refraction in Lenses: The Details

Optical Instruments

How does laser surgery provide permanent vision correction

Ans: Laser light reshapes cornea to adjust the focal point.

31.1. Images with Mirrors

Virtual

Front-to-back reversal.Right-to-left-handed coord.

Short-cut:OP = PO



GOT IT? 31.1.

You stand in front of a plane mirror whose topis at the same height as the top of your head.Approximately how far down must the mirrorextend for you to see your full image?

Ans. Half your height.

Tactics 31.1: Paraxial Ray Tracing with Spherical Mirrors

1.^

Ray // axis reflected through focus.

2.^

Ray through focus reflected // axis.

3.^

Ray striking through mirror mid-point reflects symmetrically.

4.^

Ray through mirror center reflects upon itself.

F

C

Concave Spherical Mirror

Bear and image are both in

front

of the mirror.

The Mirror Equation

Shaded triangles are similar:^ h^

s

h^

s ^

h ^

< 0

^



M

Magnification



h^

s^

f

h^

f ^

^

^

 s s 

f s

s^

^ f

^

f^

s

^

^

s^

s^

f

^

Mirror Equation

f^

R

^

R^

= radius of mirror

positive if s^

always s ,^

f^

same side of outgoing ray h ,^

h ^

above axis

Table 31.1. Image Formation with Mirrors:

Sign Conventions

Example 31.2. Jurassic Park

Convex side-view mirror:

Objects in mirror are closer than they seem.

If the curvature radius of the mirror is 12 m and the T. Rex is 9.0 m from the mirror,by what factor does the dinosaur appear reduced in size?

s ^

1

1

s



^

^

^

^

s

M

s

^

^

(image is upright & smaller)

31.2. Images with Lens

Convex lens

Concave lens

Thin lens: Light rays bend just once going through the lens.

Lens Images by Ray Tracing

Getting Quantitative: The Lens Equation

h^

s

M

h^

s ^

^

Shaded triangles similar:

h^

s^

f

h^

f ^

^

^

 s s 

f^

s^

s

^

lens equation

^

^ 

^

2

s^

f^

s^

f^

f

^

^

positive if s^

always s ,^

f^

same side of outgoing ray h ,^

h ^

above axis

GOT IT?. 31.3.

You look through a lens at this pageand see the words enlarged and right-side up.Is the image you observe real or virtual?Is the lens concave or convex?

Example 31.3. Fine Print

You ‘re using a magnifying glass (converging lens)with a 30-cm focal length to read a telephone book.How far from the page should you hold the lens inorder to see the print enlarged three times?

f^

s^

s

^

3 s

s

^

h^

s

M

h^

s

^

^

^

Image is upright enlarged so it must be virtual.Hence,

h 

0,

s 

< 0.

s^ 

^

cm 