Light Experiments at Home
Light is electromagnetic radiation in the wavelengths visible to the human eye, roughly 380 to 700 nanometers. It travels in straight lines, bounces off surfaces (reflection), bends when passing between different materials (refraction), and can be separated into its component wavelengths (dispersion). Understanding these behaviors explains rainbows, mirages, the blue sky, why objects have color, and how every optical instrument from cameras to microscopes works.
Split White Light with a Prism
White light from the sun or a bright flashlight contains all visible wavelengths mixed together. A glass prism separates these wavelengths because each color refracts (bends) by a slightly different amount as it enters and exits the glass. Violet light bends the most because it has the shortest wavelength. Red light bends the least because it has the longest wavelength. The result is a spectrum of colors spread across a surface: red, orange, yellow, green, blue, and violet.
If you do not have a glass prism, you can achieve a similar effect with a glass of water placed on a white surface near a sunny window. Angle the glass so that sunlight enters from one side and projects a spectrum onto the white paper below or beside it. A garden hose spraying a fine mist on a sunny day creates a rainbow through the same mechanism, with each water droplet acting as a tiny prism.
To demonstrate that white light is truly a combination of all colors, try recombining the spectrum. Place a second prism inverted after the first, and the separated colors converge back into white light. Alternatively, use a color wheel (a disk divided into spectrum color segments) mounted on a spinning motor. When spun fast enough, the individual colors blur together and the disk appears white, showing that mixing all visible wavelengths produces white.
Explore Refraction
Refraction is the bending of light as it passes from one medium to another with a different optical density. Place a pencil in a glass of water and observe how it appears to bend or break at the water's surface. The pencil is straight, but the light rays reflecting off the submerged portion bend as they pass from water to air, reaching your eyes from a different apparent direction than the actual position of the pencil.
Demonstrate total internal reflection by shining a flashlight or laser pointer into the side of a clear container of water at a shallow angle. Below a critical angle, light passes from the water into the air. Above the critical angle, all light reflects back into the water. This total internal reflection is the principle behind fiber optic cables, which guide light around curves by repeatedly reflecting it off the walls of a thin glass fiber.
Measure the refraction angle using a protractor. Shine a narrow beam of light (a laser pointer works best) into a flat-sided container of water at a measured angle. Mark where the beam enters the water and where it exits on the other side. Measure the angle of the beam in air and in water relative to the surface normal (a line perpendicular to the surface). Calculate the ratio of sines of these angles to find the refractive index of water, which should be approximately 1.33.
Build a Pinhole Camera
A pinhole camera (camera obscura) demonstrates how all cameras form images. Find a sturdy cardboard box and paint the inside black or line it with dark paper. Cut a small square hole in one end and cover it with aluminum foil. Use a pin to make a tiny, clean hole in the foil. Cover the opposite end with a piece of translucent material like wax paper or tracing paper, taped securely to form a screen.
Point the pinhole end toward a bright scene, such as a sunny window or an outdoor landscape. Cover your head and the screen end with a dark cloth to block ambient light. On the wax paper screen, you will see an inverted (upside-down) image of the scene outside. Light from each point in the scene travels through the pinhole in a straight line, arriving at the corresponding (but inverted) point on the screen.
Experiment with different pinhole sizes. A smaller hole produces a sharper but dimmer image because it admits less light but restricts each point in the scene to a narrower cone of rays. A larger hole produces a brighter but blurrier image because light from each point spreads across a larger area of the screen. Professional cameras solve this tradeoff by using a lens instead of a pinhole, which focuses light to produce both bright and sharp images.
Investigate Reflection
The law of reflection states that the angle of incidence equals the angle of reflection, measured from the surface normal. Verify this by shining a narrow beam of light at a flat mirror placed on a protractor. The incoming beam and reflected beam make equal angles with the line perpendicular to the mirror surface. Vary the angle and confirm that the relationship holds at every angle.
Build a simple periscope from two small mirrors and a cardboard tube or milk carton. Mount one mirror at a 45-degree angle near the top opening and another at 45 degrees near the bottom, facing the opposite direction. Light enters the top, reflects down to the bottom mirror, and reflects horizontally out to your eye. Periscopes let submarine crews see above the surface, and the same two-mirror reflection principle is used in SLR cameras to direct the image from the lens to the viewfinder.
Explore curved mirrors using a shiny metal spoon. The concave (inner) surface produces a magnified, inverted image when you hold it at arm's length and look at your face. The convex (outer) surface produces a smaller, upright image with a wider field of view. This is why convex mirrors are used for vehicle side mirrors (wide viewing angle) while concave mirrors are used in telescopes (light-gathering and magnification).
Mix Colors of Light
Color mixing with light (additive mixing) works differently from mixing paint (subtractive mixing). The three primary colors of light are red, green, and blue. When red and green light overlap, they produce yellow. Red and blue produce magenta. Green and blue produce cyan. All three primaries combined produce white light. This is how television and computer screens work, using tiny red, green, and blue subpixels to create every color you see on screen.
Demonstrate additive mixing by shining three flashlights covered with red, green, and blue cellophane or colored plastic onto a white surface in a dark room. Overlap the beams in pairs and in triples to observe the secondary colors and white. The results often surprise people who expect the colors to mix like paint. Paint mixing is subtractive because pigments absorb light, while light mixing is additive because you are combining wavelengths.
Explore how our perception of color depends on the light source by viewing colored objects under different lighting. A red apple looks red under white light because it reflects red wavelengths and absorbs others. Under pure blue light, the same apple appears nearly black because there is no red light to reflect. This experiment demonstrates that the color of an object is not an intrinsic property but depends on both the object's surface properties and the spectrum of the illuminating light.
Study Shadows and Light Intensity
Shadows demonstrate that light travels in straight lines. Place an object between a point light source (a small flashlight) and a white wall. Move the object closer to the light and observe the shadow grow larger and less sharp. Move it closer to the wall and the shadow shrinks and sharpens. This geometry, where the shadow size depends on the relative distances of the object from the light and the wall, follows simple proportional relationships you can verify with measurements.
Investigate the inverse square law of light intensity. Place a light meter (a smartphone app works adequately) at measured distances from a bare bulb or flashlight in a dark room. Measure the light intensity at 20 cm, 40 cm, 80 cm, and 160 cm. Doubling the distance should roughly quarter the intensity, because the same amount of light spreads over four times the area. Plot intensity versus the square of distance and look for the linear relationship predicted by the inverse square law.
This inverse square relationship has profound implications beyond tabletop experiments. It explains why stars appear dimmer the farther away they are, why the intensity of sunlight decreases as you move to outer planets, and why the warming effect of a heat lamp drops rapidly with distance. Understanding this single mathematical relationship connects your home light experiment to astrophysics and thermal engineering.
Light follows predictable rules of reflection, refraction, and dispersion that you can observe and measure with simple home experiments. These principles explain rainbows, cameras, fiber optics, and every visual experience you have.