Light Physics Q&A: Luminous Flux, Intensity & Equations
Light Physics and Photometry Questions
Which term describes the total amount of visible light emitted by a source per unit time? — Luminous flux
What is the unit used to measure luminous flux? — Lumen
Illuminance describes: — the luminous flux received per unit area
What is the SI-related unit used to measure illuminance? — lux
One lux is equivalent to: — 1 lumen distributed over 1 m²
Luminous intensity describes: — how much light is emitted in a particular direction
Which unit is used to measure luminous intensity? — candela
Which term refers to the process by which a light source makes objects visible? — Illumination
As light travels farther from a point source, it generally: — spreads out
According to the inverse square law, illuminance is proportional to: — 1/distance²
If the distance from a point light source is doubled, the illuminance becomes: — one-fourth as large
If the same luminous flux is spread over a larger area, the illuminance: — decreases
If the same luminous flux is concentrated over a smaller area, the illuminated surface generally appears: — brighter
Which equation represents illuminance when luminous flux and area are known? — E = Φ/A
In the equation E = I/d², I represents: — luminous intensity
In the equation E = I/d², d represents: — distance from the source
Which measurement is especially useful for directional light sources such as flashlights and car headlights? — Luminous intensity
For general room lighting, which quantities are commonly useful? — luminous flux and illuminance
A flashlight with reflectors can appear very bright in one direction because the reflectors increase its: — luminous intensity in that direction
A photon is: — a particle of light that carries energy
True or False Statements
Luminous flux is measured in lumens. — True
Illuminance is measured in candelas. — False
Lux is the unit used to measure illuminance. — True
Luminous intensity describes how much light is emitted in a particular direction. — True
Increasing the distance from a point source increases illuminance. — False
If distance doubles, illuminance becomes one-fourth of its original value. — True
Concentrating the same luminous flux over a smaller area increases illuminance. — True
A photon is a particle of light that carries energy. — True
A flashlight and a room lamp must always have the same luminous intensity if they have the same luminous flux. — False
Illumination involves the emission, travel, reflection, and reception of light. — True
Photometry Formulas and Variables
Equation: E = Φ / A
E (lx)
Φ (lm)
A (m²)
Equation: E = I / d²
E (lx)
I (cd)
d (m)
Equation: E₂ / E₁ = d₁² / d₂²
E₁ (lx)
E₂ (lx)
d₁ (m)
d₂ (m)
Practical Problems and Concept Application
1. Lamp B would appear brighter. Although both lamps have the same total luminous flux, Lamp B uses a reflector to concentrate its light into a narrow direction, which gives it a higher luminous intensity in that direction compared to Lamp A.
2. Disagree. According to the inverse square law, illuminance ($E = I / d^2$) decreases as the distance ($d$) between the light source and the surface increases. Even if the total lumens remain constant, moving the lamp farther away causes the light to spread over a larger area, resulting in lower illuminance and a dimmer desk.