8th Grade Science Study Guide: Waves & Properties of Light I. Key Concepts

1. Waves and Their Properties

• Wave: A disturbance that transfers energy from one place to another.

• Types of Waves:

  • Transverse Wave: Particles move perpendicular to the wave direction (e.g., light waves).

  • Longitudinal Wave: Particles move parallel to the wave direction (e.g., sound waves).

• Parts of a Wave:

  • Crest: Highest point of a transverse wave.

  • Trough: Lowest point of a transverse wave.

  • Wavelength (λ): Distance between two consecutive crests or troughs.

  • Frequency (f): Number of waves passing a point per second (measured in Hertz, Hz).

  • Amplitude: Height of the wave, related to energy.

Light- 

Does NOT need a medium to pass through

Behaves like a wave AND a particle

A particle of light is called a photon

• Photons have no mass and behave in a “dual nature”

Sound- 

Vibrations that travel through a medium (MATTER)

1. Explain the “dual nature” of light and the behavior of a photon.

Light exhibits dual nature, meaning it behaves as both a wave and a particle.

• Wave nature: Light exhibits properties like diffraction, refraction, and interference, which are typical of waves.

• Particle nature: Light is made up of discrete packets of energy called photons, which can eject electrons from metals in the photoelectric effect (Einstein’s discovery).

A photon is a massless particle of light that carries energy. The energy of a photon depends on its frequency.

2. Medium and Its Role in Light vs. Sound

A medium is the matter through which a wave travels.

• Light Waves: Do NOT need a medium (can travel through empty space).

• Sound Waves: NEED a medium (air, water, or solid) because they are longitudinal waves that rely on particle vibrations.

Since sound relies on particle vibrations, it moves faster in solids (where molecules are closely packed) and slower in gases. Light, being an electromagnetic wave, moves fastest in a vacuum and slows down in denser materials (e.g., glass or water).

3. By using prior knowledge of chemistry diagrams, explain the process of how photons are released.

Using the Bohr model of the atom:

1. Energy Absorption: An electron absorbs energy and moves from its ground state (lowest energy level) to an excited state (higher energy level).

2. Unstable Excited State: The electron cannot stay in the excited state indefinitely.

3. Photon Emission: The electron falls back to a lower energy level, releasing energy in the form of a photon.

4. Different Colors: The energy released determines the wavelength of light emitted. For example:

   • Red light has a longer wavelength, lower frequency, and lower energy.

   • Blue/violet light has a shorter wavelength, higher frequency, and higher energy.

4. Identify examples of different properties of light.

• Reflection: Light bounces off a surface (e.g., mirrors, still water).

• Refraction: Light bends when it passes through different materials (e.g., a straw appearing bent in water).

• Diffraction: Light spreads out after passing through a narrow opening (e.g., light bending around edges).

• Absorption: Light is taken in by a material (e.g., black clothing absorbing heat).

5. Draw and label parts of a wave based on given measurements.

• Amplitude (A): Height from the midpoint to the crest or trough (determines energy).

• Wavelength (λ): Distance between two consecutive crests or troughs (measured in meters).

• Frequency (f): Number of waves passing a point per second (measured in Hertz, Hz).

DRAW EXAMPLES OF THE WAVE using different scenarios!

6. Use the equation for speed of a wave to solve for different variables.

TO FIND FREQUENCY:

Formula - # of wavelengths/time [should be given in the example]

Frequency-Wavelength Relationship:

• Inverse relationship:

  • If frequency increases, wavelength decreases.

  • If frequency decreases, wavelength increases.

7. Explain how properties of light relate to colors on the visible light spectrum.

8. Draw the steps of light to show how the energy input affects electron movement and wavelength, frequency, and energy of photons for different colors.

1. Energy excites an electron → it moves to a higher energy level.

2. Electron falls back → releases a photon (light).

3. Photon’s wavelength depends on the energy change:

   • Small drop = lower energy, longer wavelength (e.g., red light).

   • Large drop = higher energy, shorter wavelength (e.g., violet light).

9. Apply absorption to how we see colors.

Objects absorb and reflect different wavelengths of light.

• A red apple absorbs all colors except red, which it reflects into our eyes.

• A black shirt absorbs all light (gets hotter).

• A white object reflects all colors (stays cooler).

10. Visible light spectrum relationship to wavelength, frequency, and energy.

• Red light: Longest wavelength, lowest frequency, lowest energy.

• Violet light: Shortest wavelength, highest frequency, highest energy.

• Energy is directly proportional to frequency and inversely proportional to wavelength.

11. Identify a type of wave on the full electromagnetic spectrum and its frequency and wavelength in relationship to all waves (rank them).As you move from radio waves to gamma rays:

• Wavelength decreases

• Frequency and energy increase

EXTRA CREDIT:

9. Connection to the Eye: Rods and Cones

The retina contains two types of photoreceptors:

• Rods – Detect light intensity (work in dim light, black & white vision).

• Cones – Detect color (red, green, and blue cones work in bright light).Color blindness occurs when one or more cone types don’t work properly.

10. Vision and Light Interaction

• Reflection & Refraction: Light bends as it passes through the cornea and lens, focusing on the retina.

• Absorption: Rods and cones absorb different light wavelengths.

• Transmission: Light signals travel through the optic nerve to the brain for processing.

11. Correcting Vision Abnormalities

• Nearsightedness (Myopia): Light focuses before the retina → Fixed with concave lenses.

• Farsightedness (Hyperopia): Light focuses behind the retina → Fixed with convex lenses.

• LASIK Surgery reshapes the cornea to adjust light refraction.

Final Review Questions

1. What is the dual nature of light?

2. How do photons relate to electron energy levels?

3. What is the difference between rods and cones?

4. How do we see different colors?

5. How do wavelength, frequency, and energy relate to the visible spectrum?

6. Solve: If a wave has a wavelength of 4m and a frequency of 10 Hz, what is its speed?