Physics 1230: Light and Color Ivan I. Smalyukh, Instructor Office: Gamow Tower, F Phone: Lectures: Tuesdays & Thursdays, 3:30 PM - 4:45 PM Office hours: Mondays & Fridays, 3:30 PM – 4:30 PM TA: Jhih-An Yang Class # 11

Midterm Exam Results

The remaining lectures: 3 Ch. 5 (the eye), Ch. 6 (optical instruments), Ch. 7 (Retina and visual perception), Ch. 9 & 10 (color & color perception). We are here

4 Optical instruments we’ll cover: Single lens instruments – Eyeglasses – Magnifying glass Two lens – Telescope & binoculars – Microscope We are here

5 Ray tracing a convex lens: object inside focus The image appears larger (and farther away) than the object. This is a magnifying glass. Does this match what you saw with your magnifying glass? Water drop?

6 Ray tracing a convex lens: object OUTSIDE focus The image appears smaller, inverted, and is a real image. Does this match what you saw with your magnifying glass? What can you tell me about focal length of bottle filled with water? Is it big or small?

Which person is wearing a convex lens? 7 Object close to lens appears magnifiedObject far away looks small A) The one on the left B) one on right C) Both D) ???

Nearsighted (myopic; can’t see far) = concave lenses Diverging Lens: f < 0 Farsighted (hyperopic; can’t see near) = convex lenses f>0

9 Eyeglasses: Our most common optical instrument For nearsighted people (can’t focus far away) Eyeglasses are diverging (thinner in middle) For farsighted people (can’t focus up close) Eyeglasses are converging (thicker in middle) Demo: eyeglasses Normal vision: you can focus from 25 cm to infinity (  ) 18 th century HUGE improvement in quality of life

10 1)A ray parallel to the axis is deflected as if it came from the focus 2)A ray through the center of the lens continues undeviated 3)A ray aimed at the focus on the other side comes out parallel F’ F Ray might have to be extended For diverging lens focal length defined to be negative But we’ll stick with convex lenses for ray tracing Thin concave (diverging) lens: three easy (?) ray rules

Ray tracing shows that for FARAWAY object, an image is created that is CLOSER and SMALLER than the object

Nearsighted lenses are concave: Faraway objects look closer and smaller

13 Eyeglass prescription is in diopters Optometrists use diopters to measure the power of a lens Diopters [or D] = 1 / (focal length in meters) Example: f = 50 cm or f = 0.5 m D = 1/f = 2 diopters (units are 1/meters) Does this lens fix myopia or hyperopia? Hint: Concave lenses have a negative focal length

How thin lenses add F tot = final focal length F 1 = focal length lens 1 F 2 = focal length lens 2 Diverging lenses (concave) have negative focal lengths This is the same as adding powers: D tot = D 1 + D 2 14 Demo: put together some lenses

A Question 15 You have two focusing lenses, each with a focal length of F. You put them close together to make them behave as a single lens. The new ‘doublet lens’ has a focal length of: A)2*F because the diopters add. B) F/2 because the diopters add. C) Still F for this special case. D) Something else happens. Is there an experiment you can try?

How thin lenses add F tot = final focal length F 1 = focal length lens 1 F 2 = focal length lens 2 Diverging lenses (concave) have negative focal lengths This is the same as adding powers: D tot = D 1 + D 2 16 Demo: put together some lenses OR

A Question 17 You have two focusing lenses, each with a focal length of F. You put them close together to make them behave as a single lens. The new ‘doublet lens’ has a focal length of: A)2*F because the diopters add. B) F/2 because the diopters add. C) Still F for this special case. D) Something else happens. Is there an experiment you can try? Physics is always based on the experiment!!

You have a lens with a short focal length and you wish it was longer. You can make it longer by using a second lens. The correct choice for this case is: 18 A)A focusing lens of negative power B)A diverging lens of positive power C)A focusing lens of positive power D)A diverging lens of negative power Recall:

The Magnifying glass (again): Another view 19 Typical closest focus is 25 cm from the eye. A magnifying glass is like READING GLASSES: It lets you focus on closer things. The eye perceives via focused images: 25 cm

The Microscope

21 Hooke’s discoveries The cell Detailed structure of creatures. Example: The flea (plague). From Robert Hooke’s Micrographia

22 Robert Hooke’s microscope, also circa 1660 Discovered: Blood cells, Microbes, etc. van Leeuwenhoek No existing pictures of Hooke… A TWO LENS system. Born October 24, 1632 Delft, Netherlands

23 van Leeuwenhoek’s microscope Years developed: 1660s Tiny lens with 2 mm focal length (a lens cannot be much bigger than the focal length) Magnification = Problem: image was still small, and very dim. Technology edge: Outstanding single lenses

24 van Leeuwenhoek’s microscope 25 cm, reading distance, approximately Focal length, approximately Single convex lens: Similar to a magnifying glass

Simple Microscope An unmounted and unstained blood smear Great image quality is possible!

26 Robert Hooke’s two-lens microscope object lens 1 Nosepiece or Objective lens 2 eyepiece lens 1 image lens 2 image A magnifying glass (the eyepiece) magnifies the first image further. The first lens, the nosepiece, is used as a projection lens. An image of an image!

27 Modern binocular microscope is very much the same as Hooke’s. A beamsplitter, a half- silvered mirror, sends half the light to each eyepiece.

Many optical instruments can be understood step by step, as we did for Hooke’s microscope: 28 The first lens collects light and produces an image. The second lens produces a new image of the first image. The third lens produces a new image of the second image… And so on.

Telescopes Hubble Ultra Deep Field NASA, ESA, S. Beckwith (STScl) and the HUDF Team)

31 Galileo’s telescope 30 x magnification Tiny lens means not much light entered, so image is dim. Discoveries: Sunspots Craters on the Moon Phases of Venus Moons of Jupiter

32 Galileo’s telescope (~1600) Negative lens for eyepiece gives right- side-up image. An image of an image!

33 Kepler’s telescope (~1600) Positive lens for eyepiece gives upside down image. It’s upside down, but brighter and is easier to see. lens 1 Objective lens 2 eyepiece lens 1 image lens 2 image An image of an image!

Astronomical telescope (Kepler type) astr.gsu.edu/hbase/geoopt/teles.html

Telescope kit makes a: 35 A)Galilean telescope B)Keplerian telescope C)Another type that we have not seen.

36 European Extremely Large Telescope (42 m dia.) would use many smaller mirrors

37 Telescope drives Telescopes must rotate once every 24 hrs (approximately) to follow the stars, or the pictures will have streaks. North star

Student telescopes 38 View of galaxy NGC1300

39 Catadioptric telescope Also called Schmidt-Cassegrain Front glass lens corrects aberrations Why buy this? It’s shorter.

40 Hubble Space Telescope image Compare to student telescope image.

41 Viewgraph projector Curved mirror Fresnel lens (condenser) and viewgraph location Projection lens Mirror

Slides moved to the end

A near-sighted or MYOPIC eye produces an image that is not far enough behind the lens, so is blurry on the retina. Therefore, the eye lens focal length is: 43 A)Too long for a focused image. B)Too short for a focused image. C)Actually, the iris is closed too much D)None of these.

44 Astigmatism Vertical and horizontal lines focus differently This problem is fixed by a cylinder lens Sharply focused Out of focus Focuses in one direction, but not the other!

45 Action of a cylinder lens Focuses in one direction, but not the other! If a cylinder lens is needed for your eyeglasses, your cornea and eyelens is curved more in one direction than in the other!

Reminder: Lens Power (or diopters) Lens power: D = 1/F Units of D are 1/meters, also called diopters Eyeglass lenses are measured in diopters.