Reflection, Refraction, and Color

A mirror redirects light, a glass block bends it, and a prism spreads colors because light interacts with boundaries and materials in predictable ways.

This lesson supports Chapter 21 of Physics for Beginners. Keep the physical picture in view, write the units, and check the direction of every vector before trusting a calculation.

Reflection follows equal angles

The angle of reflection equals the angle of incidence, with both angles measured from the normal line. A plane mirror forms an upright virtual image that is the same distance behind the mirror as the object is in front.

Refraction follows Snell’s law

When light crosses a boundary, its speed and wavelength can change. Frequency stays the same. Snell’s law is \[n_1\sin\theta_1=n_2\sin\theta_2.\] Light bends toward the normal when it enters a medium with a larger refractive index.

Color depends on spectrum and response

White light contains a range of visible wavelengths. Dispersion occurs because refractive index varies slightly with wavelength. An object looks red under white light because it reflects red wavelengths strongly and absorbs much of the rest. Additive color mixing uses light, while subtractive mixing uses pigments or filters.

Worked example

Light moves from air into glass. Because glass has the larger refractive index, Snell’s law predicts that the refracted ray bends toward the normal. The frequency does not change at the boundary, but speed and wavelength decrease.

Watch the idea in action

Professor Dave Explains gives a focused explanation of the chapter’s central idea. Pause before each calculation and predict the next step on paper.

Practice questions

1. State the law of reflection.
2. From which line are optical angles measured?
3. Write Snell’s law.
4. Which way does light bend when entering a higher-index medium?
5. Does frequency change at a boundary?
6. Why does a prism spread white light?

Answers

1. The reflection angle equals the incidence angle.
2. The normal.
3. \(n_1\sin\theta_1=n_2\sin\theta_2\).
4. Toward the normal.
5. No.
6. Refractive index depends slightly on wavelength.

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