Light Flashcards
What are the main properties of light?
- Light is an electromagnetic wave.
- It does not need a medium to transfer (it can travel through a vacuum).
- It is a transverse wave (energy transfer is perpendicular to wave direction).
- It transfers energy from one point to another.
- It can reflect, refract, and diffract.
- Its speed in air/vacuum is 3×10^8 m/s.
- It consists of 7 colors (ROYGBIV).*
What is monochromatic light, and how does it compare to polychromatic light (white light)?
- Monochromatic light has one frequency/wavelength.
- Polychromatic (white) light contains multiple frequencies (the full ROYGBIV spectrum).
State the law of reflection in a plane mirror and list the image properties.
Law of reflection: Angle of incidence = Angle of reflection.
Image properties in a plane mirror:
* Upright (erect).
* Same size as the object.
* Virtual image.
* Laterally inverted.
* Distance from object to mirror = distance from mirror to image.
What is the difference between a real image and a virtual image?
- Real image: Can be formed on a screen; formed by actual rays coming from the object.
- Virtual image: Cannot be formed on a screen; formed by rays that only appear to come from the object.
Define refraction and explain why it occurs.
- Refraction is the change in direction of a wave as it passes from one medium to another.
- It happens because the wave’s speed and wavelength change in the new medium, causing the direction to bend.
What is Snell’s law for refraction from air to a medium and from a medium to air?
- Air to medium: 𝑛 = sin(𝑖) / sin (𝑟)
- Medium to air: 𝑛 = sin(𝑟) / sin (𝑖)
Define the terms 𝑖, 𝑟, and 𝑛, and explain how to calculate 𝑛
- 𝑖 = angle of incidence.
- 𝑟 = angle of refraction.
- 𝑛 = refractive index, which is the ratio: speedoflightinair / speedoflightinmedium
- A medium with a higher 𝑛 has lower light speed.
- In formula form: 𝑛 = (3×10^8) / 𝑣
What happens when light passes from air (rarer) into a denser medium (Case 1)?
- 𝑛(1) < 𝑛(2) and 𝑣(1) > 𝑣(2) || (fast to slow).
- The ray bends towards the normal (𝑖 > 𝑟)
- Use 𝑛 = sin(𝑖) / sin(𝑟)
.
What happens when light passes from a denser medium into air (rarer) (Case 2)?
- 𝑛(1) > 𝑛(2) and 𝑣(1) < 𝑣(2) || (slow to fast).
- The ray bends away from the normal (𝑖 < 𝑟)
- Use 𝑛 = sin(𝑟) / sin(𝑖)
What is the critical angle (Case 3) and how is it defined?
- The critical angle 𝑐 is the angle of incidence in a denser medium that produces a 90° angle of refraction in air (𝑟 = 90∘).
- 𝑖 = 𝑐: means the refracted ray just grazes the surface.
- 𝑛 = 1 / sin(𝑐)
Explain total internal reflection (Case 4) and when it occurs.
- Total internal reflection (TIR) happens when 𝑖 > 𝑐 (angle of incidence is greater than the critical angle) and light travels from a denser medium to a rarer medium.
- All light is reflected inside; no refraction leaves the medium.
What occurs when a light ray passes normally (i = 0°) to a surface (Case 5)?
The ray travels straight through without bending because there is no angle of incidence.
How do a reflecting prism and a parallel glass plate affect light rays?
- Reflecting prism: Can use total internal reflection or angled reflection (e.g., in a periscope) to change the light path.
- Parallel glass plate: The angle of incidence on the first surface equals the emerging angle from the second surface, making the incident ray parallel to the emerging ray.
What is special about a semicircular glass plate in refraction experiments?
Any ray incident on the curved (semicircular) surface at its center enters normally (Case 5), so it passes straight through without deviation.
How do optical fibres use total internal reflection?
- Optical fibres have a high 𝑛 interior layer and lower 𝑛 exterior layer.
- Light entering at suitable angles reflects internally (TIR) along the fibre without escaping.
How are optical fibres used in medicine?
- They are used in endoscopes for medical imaging.
- A fibre is inserted into a blood vessel or body cavity to reach an organ.
- Light travels inside the fibre by TIR to illuminate the organ and returns the image by TIR to a screen.
How are optical fibres used in communication?
- They carry telephone, internet, and TV signals at high data rates with more security over longer distances.
- TIR ensures minimal signal loss.
What is dispersion of light, and what happens to white light in this process?
- Dispersion is the splitting of white (polychromatic) light into its 7-color spectrum (ROYGBIV).
- Different wavelengths refract by different amounts, causing the colors to separate.
Which color has the longest wavelength and highest speed in a medium, and what does that cause?
- Red has the longest wavelength and thus the highest speed in a medium.
- It refracts the least and emerges first in dispersion (ROYGBIV order).
What is a convex (converging) lens, and how do you construct its ray diagrams?
A convex lens bends light rays inward, focusing them.
Ray diagram steps:
1. A ray parallel to the principal axis refracts through the focus.
2. A ray through the center of the lens continues undeviated.
3. A ray through the focus refracts parallel to the principal axis.
What is near-sightedness (short sight), and how is it corrected?
- A near-sighted person can see near objects clearly but not far objects.
- The image forms in front of the retina.
- It is corrected with a diverging (concave) lens.
What is far-sightedness (long sight), and how is it corrected?
- A far-sighted person can see far objects clearly but not near objects.
- The image forms behind the retina.
- It is corrected with a converging (convex) lens.
List the common properties of all electromagnetic waves.
- They travel at the speed of light in vacuum (3×10^8 m/s).
- They can propagate through a vacuum (no medium required).
- They are transverse waves.
What is the order of the electromagnetic spectrum from longest to shortest wavelength?
Radio waves → Microwaves → Infrared (IR) → Visible light → Ultraviolet (UV) → X-rays → Gamma (γ) rays.
How are radio waves used, and why can they pass through walls easily?
- Radio waves are used in non-satellite communications (TV, radio) and Bluetooth.
- Their long wavelength and low frequency let them penetrate walls.
How are microwaves used, and what adverse effect can they have?
- Uses: Cooking (microwave ovens), satellite communications, mobile phones.
- Adverse effect: Heating effect on tissues.
What are the uses and adverse effects of infrared (IR)?
- Uses: Remote controls, security alarm systems, night vision cameras.
- Adverse effect: Can cause skin burns.
What are the uses and adverse effects of ultraviolet (UV) radiation?
- Uses: Sterilization of medical equipment, hot missile tracking, security marking, sterilization of water.
- Adverse effects: Skin cancer, blindness/damage to retina.
How are X-rays and gamma (γ) rays used, and what are their adverse effects?
- X-rays: Bone photography, airport security, detecting hidden weapons.
- Gamma rays: Security systems, special cameras, sterilization of food.
- Both: Can cause cancer, mutations, and kill cells.
What is the difference between analogue and digital signals, and why are digital signals advantageous?
- Analogue signals: Continuous variation, prone to noise.
- Digital signals: Two-step voltages (0 = low, 1 = high), can be regenerated accurately, reduce noise, allow higher data transfer rates, and can be converted from analogue.
What is diffraction, and when is it most significant?
- Diffraction is the spreading of waves when they pass through a gap or around an obstacle.
- It is most significant when the gap size is similar to the wavelength.
- Longer wavelengths (e.g., radio waves) diffract more than shorter ones (e.g., light).
What is white light composed of, and how does dispersion separate it?
- White light is a combination of all visible colours (ROYGBIV).
- Dispersion occurs because different wavelengths refract by different amounts when passing through a prism.
Describe how light behaves when it refracts from air into a denser medium.
When light passes from air to a denser medium, its speed decreases, the wavelength shortens, and it bends towards the normal, while its frequency remains constant.
Which type of lens is used for near-sightedness (myopia) and why?
A diverging (concave) lens is used to correct near-sightedness because it moves the image from in front of the retina onto it.
What is the standard method for drawing ray diagrams for lenses?
- Draw one ray parallel to the principal axis (which refracts through the focal point)
- Draw one ray through the centre (which is undeviated)
- Draw one ray through the focal point (which refracts parallel to the principal axis)
What are the advantages of digital signals over analogue signals in communication systems?
Digital signals are less prone to noise, can be regenerated without quality loss, and support higher data transfer rates, leading to more reliable long-distance communication.
