3 Properties of Laser Light
Properties of Laser Light
This is a three part investigation into the properties of laser light – Monochromatic, Collimated and Coherent.
Grades: 9 to 14
Duration: 1/2 Hour – 1 Hour
Materials and supplies
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Properties of Laser Light: Monochromatic
First, we will explore the monochromatic nature of laser light by comparing the spectra of white light and colored LEDs to that of LASER light.
Step 1: Observe the spectra of white light
Look through the diffraction grating towards the white LED. Don’t look directly at the light; rather slightly to the left of right of the source.
What do you see? Record the order of the colors, starting with the color closest to the light source.
White light consists of all the colors of the rainbow – it is made up of many wavelengths of light so it is NOT monochromatic. The “rainbow” observed through the diffraction grating are the wavelengths of light that make up that white light.
Now hold the diffraction grating up to a flashlight, the overhead lights, (NEVER THE SUN).Was the rainbow the same for all sources of white light? What do you expect to see for the colored light from the Light Blox?
Step 2: Observe the Spectra of colored light
Repeat the process with the red, blue and green Light Blox. Do you expect to see only red light or a spectrum?
What do you see? Was this what you expected? Why/why not? What’s going on?
This image is of the spectrum generated by a red LED. You can see that there are several wavelengths of light present. Its not the full “rainbow” you see with white light, but clearly the LED is not monochromatic (consisting of only a single wavelength)
Step 3: Observe laser light through a diffraction grating
Point the red LASER pointer towards a blank wall or screen, approximately 15-20 cm away from the surface.
Hold the diffraction grating about 5-10 cm from the front of the LASER, and switch on the LASER pointer so that the laser beam passes through the diffraction grating onto the wall.
What do you see? How is this different from the light from a Light Blox? What does this tell us about what the properties of laser light are? If you have a green laser pointer, try it with green and see what happens.
What you are seeing here is the diffraction pattern of monochromatic (single wavelength) red laser light. Because there is only one wavelength (RED, 635nm) you only see the beam splitting into many red dots, not a spectrum; there are no other colors (wavelengths) present.
So Monochromatic light is light of a single wavelength or color. Lasers produce monochromatic light.
Properties of Laser Light: Collimated
This activity will showcase another of the properties of LASER light that makes it so unique. Another property of Laser light is that it isCollimated.
Step 1: Set up
Stick one a blank sheet of paper against the wall, in line with the table top. Next, stick one a blank sheet of paper against the wall, in line with the table top, like in the picture here.
Hold a RED Light Blox 1” from the paper on the wall, and switch it on. Use a pen or pencil to trace the area of the rectangle of light created. Repeat at 3” and 6”.
Step 2: Collect, calculate and record your data
Remove the paper from the wall and calculate the area of each of the rectangles. Down load the lesson at the beginning of this post and record your data in the table on the worksheet.
Repeat steps 4 and 5 with the red LASER pointer.
What do you notice about the size of the rectangle from the Light Blox at various distances from the wall? What do you think this might mean about the light produced? It changes – the rectangle gets smaller as the light gets closer to the wall, and larger when it gets further away.
Non-Collimated light spreads out as it travels.
What do you notice about the size of the circle/ellipse of laser light at various distances from the wall? It does NOT change. What could this mean about how laser light is different from ordinary light?
Therefore, collimated light travels long distances without spreading out. This is why we have a laser BEAM.
Properties of Laser Light: Coherent
Coherent is the most challenging of the three properties of laser light to understand. In this activity we’ll demonstrate that LASER light is coherent by comparing the interference pattern of a red LASER pointer to that of an LED light.
Step 1: Observe SPECKLE
Switch on the Light Blox and hold it close to the paper, at a small angle. The light should stretch out in front of the Light Blox. Take a close look at the light.
What do you observe? Is there anything unusual about the light?
Now, hold the LASER pointer the same way, and switch it on. The LASER light should form a long ellipse.
What do you observe about the LASER light? Is there anything unusual happening? How is it different from the light from the Light Blox?
You might have to move between the Light Blox and the LASER pointer a couple of times before you see the difference in the light – keep going until you see it!
Now that you know what the speckle pattern looks like, move the LASER slowly while shining it on the paper.
What do you observe about the speckle pattern? Why do you think it happens?
Speckle is a random granular pattern that is observed when a highly coherent beam (laser) is diffusely reflected off a rough surface. The speckle patter is actually an intensity pattern that is produced by the interference of the laser beam’s wavefronts.