PHYSICS 124 EXPERIMENT NO. 8PHYSICS 124 EXPERIMENT NO. 8REFLECTION, REFRACTION, LENSE AND

OPTICAL INSTRUMENTS

Goal:Goal: The purpose of this laboratory is to study the Laws of Reflection and Refraction, The purpose of this laboratory is to study the Laws of Reflection and Refraction, to measure the index of refraction of glass and observe dispersion. Then you investigate to measure the index of refraction of glass and observe dispersion. Then you investigate images of lenses.images of lenses.

Introduction:Introduction:This laboratory is to show that the very simple principles of reflection and refraction can This laboratory is to show that the very simple principles of reflection and refraction can lead to sophisticated ideas about optical instrument. We begin with a ray box that has a lead to sophisticated ideas about optical instrument. We begin with a ray box that has a slotted mask in front of a light bulb to produce a set of thin beams (or "rays"). The rays slotted mask in front of a light bulb to produce a set of thin beams (or "rays"). The rays lie along a plane surface (a sheet of paper). Your measurements will consist of pencil lie along a plane surface (a sheet of paper). Your measurements will consist of pencil mark on a piece of paper lying on this surface, indicating the direction of these rays mark on a piece of paper lying on this surface, indicating the direction of these rays before and after striking a mirror or glass prism. Then you will progress to the study of before and after striking a mirror or glass prism. Then you will progress to the study of lenses, which are nothing more than refractor with curved surfaces.lenses, which are nothing more than refractor with curved surfaces.

Part I-Light Wave Refection:Part I-Light Wave Refection:

Equipment:Equipment:1 “Ray” table1 Triangular prism

Fig 1.The slit that the light comes through creates multiple rays. You should find a way to The slit that the light comes through creates multiple rays. You should find a way to block all but the central one.block all but the central one.

A. Use the ray box, a mirror, and protractor to verify that θA. Use the ray box, a mirror, and protractor to verify that θincidentincident = θ = θreflectedreflected (or θ (or θincinc = = θθreflrefl). Do this for at least three different incident angles.). Do this for at least three different incident angles.

Table with 8.5x11” paperTable with 8.5x11” paperfor “ray” tracingfor “ray” tracing

LampLamp(sliding) on rails)

Slits create light beamsSlits create light beams(all but one covered) (all but one covered)

Triangular prismTriangular prism

B. Show that the (virtua1) image produced by B. Show that the (virtua1) image produced by a a mirror mirror is is located located as as far behind the mirror far behind the mirror as as the object the object is in is in front of the mirror. Do this by marking the point that the rays start at, front of the mirror. Do this by marking the point that the rays start at, the point that the ray hits the mirror and a point out along the reflected beam. Do this for the point that the ray hits the mirror and a point out along the reflected beam. Do this for 3 beam angles. Measure the distance between the mirror and the point that the rays start 3 beam angles. Measure the distance between the mirror and the point that the rays start at, then trace back the reflected rays until they meet and measure the distance between at, then trace back the reflected rays until they meet and measure the distance between that point and the mirror and record those two distances in your lab book.. The two that point and the mirror and record those two distances in your lab book.. The two distances should be about the same.distances should be about the same.

Part II- Light Wave RefractionPart II- Light Wave Refraction

A. Snell's LawA. Snell's LawUse the ray box, the glass triangle and protractor to verify Snell's Law about refraction:Use the ray box, the glass triangle and protractor to verify Snell's Law about refraction:

nnincinc sin θ sin θincinc = n = nreflrefl sin θ sin θreflrefl

Where nWhere nincinc and n and nrefl refl are the refractive indices of the incident and refracting media are the refractive indices of the incident and refracting media respectively. Take nrespectively. Take nair air = 1 and n= 1 and nglass glass = 1.5. Verify the law by finding the ratio of the sines = 1.5. Verify the law by finding the ratio of the sines for at least two different angles θfor at least two different angles θincinc. There is a picture below of what you should draw in . There is a picture below of what you should draw in your lab book. The yellow line is the light beam and the black line is perpendicular to theyour lab book. The yellow line is the light beam and the black line is perpendicular to the side of the prism. Theta1 is θside of the prism. Theta1 is θincinc and theta2 is θ and theta2 is θreflrefl..

B. Critical anglesB. Critical anglesSnell's law also tells us that if we reverse things, i.e., let light hit a glass-air boundary Snell's law also tells us that if we reverse things, i.e., let light hit a glass-air boundary thenthen

Now sinθNow sinθreflrefl can never be greater than 1, and is maximal for θ can never be greater than 1, and is maximal for θreflrefl = 90 = 90o o the angle θthe angle θinc inc (this is (this is now on the inside of the glass, not the angle the outside of the glass is hit at) where this now on the inside of the glass, not the angle the outside of the glass is hit at) where this happens is given the name critical angle since for any θhappens is given the name critical angle since for any θincinc > θ > θcriticalcritical, the sine would have to , the sine would have to greater than 1. Since this can not be, light must be trapped inside the glass, it must be greater than 1. Since this can not be, light must be trapped inside the glass, it must be totally reflected. Use your prism to find θtotally reflected. Use your prism to find θcriticalcritical. Compare your measured angle with what . Compare your measured angle with what you expect on the basis of the equation above.you expect on the basis of the equation above.

C. DispersionC. DispersionUse the prism to observe light dispersion. Wavelength has a small effect on the index of Use the prism to observe light dispersion. Wavelength has a small effect on the index of refraction and so different colors will bend at different angles. To see this get the light to refraction and so different colors will bend at different angles. To see this get the light to reflect internally once before it leaves and have the beam leave close to parallel to the reflect internally once before it leaves and have the beam leave close to parallel to the side of the prism. The further the beam travels out of the prism the more you will be ableside of the prism. The further the beam travels out of the prism the more you will be able to see the spread. You should mark both edges of the spread in the beam marking which to see the spread. You should mark both edges of the spread in the beam marking which side is red and which is blue. side is red and which is blue.

Q1. Which colors are bent the most? Q1. Which colors are bent the most?

From measured angles, deduce nFrom measured angles, deduce nblueblue and n and nred red for your prism.for your prism.

Part III ---LensesPart III ---Lenses

Equipment:Equipment:1 “Optical Bench: a rail to mount the holders of lenses/screen on1 Box with lenses (Use #3 and # 4 only – marked at bottom of shaft) 2 Holders for lenses/screen1 Screen1 Lamp with arrow (our object to be imaged)

Fig. 3The apparatus consists of an optical bench which serves as a convenient holder for The apparatus consists of an optical bench which serves as a convenient holder for objects, lenses and a ground glass screen for locating images. The object is an arrow objects, lenses and a ground glass screen for locating images. The object is an arrow painted on a piece of ground glass illuminated from behind by a collimated light bulb.painted on a piece of ground glass illuminated from behind by a collimated light bulb.

Measure the focal point of the 5cm converging lens (the one marked with a 4 on Measure the focal point of the 5cm converging lens (the one marked with a 4 on the bottom) and compare your number to the marked focal length. To do this set put the the bottom) and compare your number to the marked focal length. To do this set put the painted arrow close to the lamp, fix the screen some distance away from the arrow and painted arrow close to the lamp, fix the screen some distance away from the arrow and then move the lens until the image on the screen is most focused. At this point lock the then move the lens until the image on the screen is most focused. At this point lock the

RailRail

HoldersHolders

ScreenScreen(Image)(Image)

Box with lenses: useBox with lenses: use#4: #4: ff =5 =5 cmcm, converging, converging#3: #3: ff =10 =10 cmcm, diverging, diverging

LampLamp

Arrow(Object)Lens frameLens frame

lens into place and measure the distance from the arrow to the lens (xlens into place and measure the distance from the arrow to the lens (x00) and the distance ) and the distance from the lens to the image (xfrom the lens to the image (xii). The focal length is found by the formula). The focal length is found by the formula1/f = 1/x1/f = 1/x00 +1/x +1/xii..Be careful here because the lens is not in the center of the holder, so you should measure Be careful here because the lens is not in the center of the holder, so you should measure the distance from the center of the lens-holder to the center of the lens and then either addthe distance from the center of the lens-holder to the center of the lens and then either add or subtract to the distance to get the correct value of xor subtract to the distance to get the correct value of xoo or x or xii..

Write-Up:Write-Up:Include the following in your lab report:Include the following in your lab report:

a)a) Answer question 1.Answer question 1.b)b) Verify that θVerify that θincidentincident = θ = θreflectedreflected

c)c) Verify that the image is the same distanceVerify that the image is the same distanced)d) Verify Snell’s LawVerify Snell’s Lawe)e) Calculate critical angle and compare to what is expected from the Calculate critical angle and compare to what is expected from the

formulaformulaf)f) Find nFind nblueblue and n and nredred..g)g) Find the focal length of the two lenses and compare to their labeled Find the focal length of the two lenses and compare to their labeled

value.value.