Light travels in straight lines until something makes it turn. Today you’ll find the rule that governs how it bounces off a mirror, locate the image in a flat mirror, and see what happens when the mirror curves.
🎯 Goals
Test the law of reflection with a beam of light and a protractor.
Locate the image in a plane mirror with a ray diagram, and describe it.
Explore concave and convex mirrors with a spoon, and predict how their images look.
Use the law of reflection to solve angle problems, find a mirror’s needed height, and explain a periscope.
🧰 Materials
🪞A small plane (flat) mirror, set upright in a lump of modeling clay
🔦A laser pointer or ray box. Pins and cardboard also work (see the note in Section 1)
📐A protractor and a ruler
📄Plain paper, taped to a flat board or cardboard
🥄A shiny metal spoon, clean, with a polished bowl
✏️A sharp pencil and a few colored pencils
🧮Calculator
📓Physics notebook
Section 1 of 4
The Law of Reflection
1
Set up the mirror and draw the normal.
Tape a sheet of paper to a board. Draw a straight line across it and stand the mirror upright on the line, with its shiny face toward you, held in place by a lump of clay. At the middle of the mirror, draw a line at exactly 90° to the mirror. This is the normal. Every angle in this activity is measured from the normal.
The law of reflection: θ_reflection = θ_incidence Both angles are measured from the normal, the line at 90° to the mirror, and the incident ray, the reflected ray, and the normal all lie in the same flat plane.
2
Trace five rays.
Aim a narrow beam along the paper so it strikes the mirror where the normal meets it. Use angles of incidence of 0°, 20°, 30°, 45°, and 60° from the normal. For each, mark two dots along the incoming beam and two along the reflected beam, then remove the mirror and join the dots with a ruler. Measure the angle of reflection from the normal.
⚠️ Never point a laser at anyone’s eyes, and keep the beam flat against the paper. Reflections can be as bright as the beam. No laser? Stand two pins along the incident ray, then sight along the mirror and line up two more pins with the pins’ images. Draw the rays through the pin holes.
3
Record your angles.
Fill in Table A, then use the Reflection Checker on any row to see whether your angles agree within a protractor’s reading error.
Enter one row of your data to see whether your two angles agree. A protractor can only be read to about ±2°.
What good data looks like
Your reflected angles should match your incident angles to within about 2–3°. Differences that are sometimes positive and sometimes negative are just reading error. Differences that always go the same way, especially large ones, usually mean the angles were measured from the mirror instead of the normal, or the mirror wasn’t on its line.
📓 Physics Notebook
State the law of reflection in your own words, and say how well your data support it. What would a differences column look like if the law were wrong?
Section 2 of 4
Where Is the Image?
4
Find the image with a ray diagram.
When you look in a mirror, your brain assumes light travels in straight lines, so it traces the reflected rays backward to where they seem to come from. That spot is the image. Use the Mirror Ray Diagram below. The dashed lines show the backward trace, and they meet at the tip of the image.
Mirror Ray Diagram
Two rays leave the tip of the object and bounce off the mirror by the law of reflection. Your eye follows the reflected rays backward in straight lines (the dashed lines), and they meet at the tip of the image.
Object: d_o = 6 cm, h_o = 3 cm Image: d_i = 6 cm behind the mirror, h_i = 3 cm, upright No light actually passes through the image, so it is a virtual image.
5
Record three object positions.
Set the object at three different distances and heights. Record the object distance and height, and read the image distance and height from the diagram.
Object distance d_o (cm)
Object height h_o (cm)
Image distance d_i (cm)
Image height h_i (cm)
📓 Physics Notebook
Describe the image in a plane mirror: how far behind the mirror is it, how big is it, and which way up? Why is it called a virtual image? (Could you catch it on a sheet of paper placed behind the mirror?)
Section 3 of 4
Curved Mirrors: The Spoon Test
6
Look into the bowl of the spoon.
Hold the spoon with its inside (the concave side) facing you, at arm’s length, and look at your reflection. Slowly bring it closer to your face. Notice what happens to your image, including when it flips.
7
Turn the spoon over and repeat.
Now use the back of the spoon (the convex side), at arm’s length and then closer. Record what you see in Table C.
Spoon side
How far from your face
Upright or inverted?
Bigger or smaller than you?
8
Predict the image for four mirrors.
Use your table to choose the orientation and size of the image for each case.
Predict the Image
Use what you saw in the spoon. For each mirror, choose how your image looks.
A plane (flat) mirror
A concave mirror, with the object far away
A concave mirror, with the object very close
A convex mirror, at any distance
📓 Physics Notebook
Which side of the spoon would make a good makeup mirror, and which would make a good store security mirror? Explain each in a sentence using what you saw. Then, in your own words, answer the unit’s third question: why can a curved mirror make you look bigger, smaller, or upside-down depending on where you stand?
Section 4 of 4
Put the Law to Work
9
Solve six mirror problems.
Use θ_r = θ_i, measured from the normal. The last two connect to real things: a wall mirror and a periscope. A correct answer is within 2%.
Mirror Problems
Angles are in degrees, and heights are in meters.
A ray hits a plane mirror with an angle of incidence of 35°. What is the angle of reflection?
A ray makes a 25° angle with the surface of a mirror, not with the normal. What is its angle of incidence?The normal is 90° from the surface
A ray strikes a mirror 40° from the normal. What is the angle between the incident ray and the reflected ray?
The incoming ray stays fixed and you rotate the mirror by 10°. By how many degrees does the reflected ray turn?The normal turns 10° too
How tall must a wall mirror be for a 1.70 m person to see their whole body in it?Think about where the head and foot rays hit the mirror
A periscope uses two parallel mirrors, each at 45° to the light path, and light enters horizontally. What is the angle between the entering direction and the exiting direction?
📓 Physics Notebook
Sketch a periscope that lets you see over a wall. Draw the two mirrors, the path of a ray from an object on the far side of the wall to your eye, and label each angle of incidence and reflection.
Question 1
Use your Table A data to state the law of reflection, and say how well your measurements support it.
Question 2
Describe the image in a plane mirror, including how far behind the mirror it is. Explain why it is called a virtual image.
Question 3
Using the spoon and the Predict the Image tool, explain why a curved mirror can make you look bigger, smaller, or upside-down depending on where you stand.
Question 4
Explain how two plane mirrors let you see over a wall. What angle does each mirror make with the light path?
📤 Turn In
Completed Table A (angles) and your notebook statement of the law
Completed Table B (image positions) and your description of a plane-mirror image
Completed Table C (spoon observations) and your Predict the Image results
Your Mirror Problems results and your periscope sketch