Light

Vanderbilt Student Volunteers for Science

Fall 2011

Safety

• Tell the students that they will be using lasers to study some properties of light waves, hence several rules that MUST be followed:– Be very careful with the laser pointer. – Never aim it at anyone. – When turning it on, always have it pointed

away from your eyes and from other persons. Eye damage can occur with direct eye exposure to the laser beam.

I. Introduction

• Ask students what they know about light.

• Points to take note during discussion:– Light is energy emitted by vibrating electric

charges.– Light is electromagnetic wave. – Visible light consists of wavelengths from 400

to 700 nm. (A nanometer is 1x10-9 meters)

IIa. Demo: Light Scattering Using Particulate Matter

• Obtain the bottle of flour from the kit.

• Shine the laser light towards a wall away from the students.

• Compress bottle steadily and lightly so that the flour drops through the path of the laser light in a constant stream.

• Students should be able to see the laser light reflected upon the particles of flour.

IIb. Scattering

• Tell the students to place the vial of liquid upright on the desk.

• Tell them to point the laser to the side of the vial. They may see the laser beam enter and exit the vial at the glass edges, and will see the red beam hit the table. They will not be able to see the ray traveling through the water

• Tell students to gently shake the second vial containing a few drops of Milk of Magnesia, so that the liquid is cloudy and repeat the above step. Now they will be able to see the laser beam passing through the liquid.

Red Dot

IIb. Scattering

• The white milky substance added

to the second vial is milk of

magnesia, which is made of very

small white particles. These

particles scatter the light to your

eye so you can see it.

• Turn vial so that it’s horizontal and

shine laser at the bottom. The

beam does not go all the way to

the other end. It gets weaker the

further across the vial it travels.

III. Reflection

• Place the block of wood with the mirror on the marked line on the observation sheet. The mirror must be flush with the sheet and line up EXACTLY with the black line.

• Designate one student to hold the laser pen. Remind the students to NEVER look directly into a laser beam.

• Place the laser in the groove of the ruler and place it so that the beam from laser shines along the solid 45° line and is pointed to the “X”

III. Reflection

• Hold a popsicle stick in the path of the laser beam, so that the red dot is visible on the stick. Have the students “trace” the laser beam along the 45° line in towards the mirror. Adjust the position of the laser if necessary. (The laser can be tilted down so that the beam shines on the paper.)

• Trace the reflected beam with the popsicle stick and to note which line the stick moves along. (It should be close to the dotted 45° line.)

• Tell the students that the light from the laser to the mirror is called the incident ray and the light from the mirror is the reflected ray.

• Explain that when light goes in at an angle on one side (left or right) it comes out at the same angle on the other side. (Draw this concept on a board)

• Remember: The concept the students should learn is that light can bounce or reflect. Light goes in at one angle and comes out on the opposite side at an equal angle.

• Allow the students to try other angles (moving the ruler and laser) to see what happens. Remind students to aim for the “X” in the center.

• COLLECT ALL LASER PENS, COUNT AND PLACE IN BOX. DO NOT GO UNTIL YOU HAVE PLACED THE LASER POINTERS IN THE VSVS BOX.

IVa. Diffraction“Looking Through a Diffraction Grating”

Note: A VSVS volunteer should demonstrate the proper use of a diffraction grating slide.

• Tell students to hold the slide by the cardboard only.

• Do not touch the clear film in the cardboard holder.

• Hold the diffraction grating close to (but not touching) the eye and look at any lights or windows in the room.

• Several rainbows should appear.

• CAUTION: DO NOT LOOK DIRECTLY AT THE SUN THROUGH A DIFFRACTION GRATING

IVb. Diffraction“CD”

• Tell the students to pick up the CD and notice the “rainbow” pattern from the room lights.

• Explanation: CDs have many parallel grooves so the CD acts as a diffraction grating.

IVc. Diffraction“Lasers and Diffraction Gratings”

• Standing several feet from the wall, hold the

laser light close to the diffraction grating

slide.

• Shine the laser light through the diffraction

grating to show three red points of light on

the wall. The red dots will be in a

horizontal, vertical, or diagonal line

depending on the way you hold the

diffraction grating slide. Rotate the slide to

see the dots change position.

• Shine a green laser through another grating

so that center of green and red dots

overlap.

– Green light should have spread out further

than red ones. Green diffracted differently

and all wavelengths are diffracted at different

rates.

Va. Total Internal Reflection and Fiber Optics

Light Pipe: Demonstration• Hold the light pipe so that the long part

is vertical and the small horizontal part is pointing towards the class (but not directly at any person’s eyes).

• Shine the red laser beam up towards the ceiling and have the students notice the red color on the ceiling.

• Shine the laser through the bottom end of the pipe.

• Show students that the red light can be seen at the horizontal end, and that no light escapes to the ceiling. The red light can be seen traveling around the tube.

• Explanation: When the angle of incidence is high enough (above a critical angle characteristic of the substance; 42° for glass), the incident light is totally reflected inside the medium

Vb. Total Internal Reflection and Fiber OpticsFiber Optic Cables

• Tell students that fiber optics are often referred to as "light pipes”. Light remains inside of the transparent fibers by means of total internal reflection.

• Shine the laser light through the fiber optic cable to show that the red laser light can be transmitted by fiber optic cable.

• Explanation: Light can be "piped” from one location to another in fiber optic material. On entering the "light pipe” at an angle greater than the critical angle of pipe material, the light undergoes repeated internal reflections and follows the contour of the pipe.

Vb. Total Internal Reflection and Fiber OpticsFiber Optic Cables

Have one student in each group turn on the fiber optic flashlight so the students can observe fiber optics cables for themselves.

Tell them to notice that the light can be seen only at the ends of each fiber.

• Some students may see different colors – this is because there are colored filters inside the bowl of a few of the flashlights.

V. Clean-Up

• Make sure to collect all the lasers and count them.

– Check to make sure that the laser pointers are not left on in the cases. If one is on in the case, open it and rotate the pointer so the button is to one side, not straight up, as this will cause the lid of the case to press the button.