# Speed of Light from Car Headlights & Brake Lights

• I
• Tony Sims
In summary: In the same way, regardless of how fast you move, the speed of light will always move away from you at speed ##c##. This is the absolute speed limit.But what about the speed of light from the brake lights pointing backwards?This is a bit more complicated. If you're an observer standing behind the car, and the car is moving along the road, you will always measure the same value ##c## for the speed of the light coming from the brake lights. This is equivalent to the car being at rest and you moving towards or away from it. Therefore no matter how fast you move, you will always measure the same value ##c## for the speed of light.However, if you're an
Tony Sims
So there is lots of information on the Internet specifying that the speed of light from the headlights of a car traveling at 30mph is C and not C+30 because it is impossible to travel faster than light but what about the speed of light from the brake lights pointing backwards? Am I right in thinking that this would be C-30 or is it still C?

Welcome to PF!

The speed of light measured by any inertial (non-accelerating) observer in a vacuum is C. Just C. Always C.

There is probably a related question you will think to ask soon which relates to how you add velocities for objects. For example, say you have, two rocket ships on opposite sides of Earth traveling toward Earth at 0.8 C. Do they measure their speed relative to each other to be 1.6C? Nope:
http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/einvel.html

Tony Sims said:
So there is lots of information on the Internet specifying that the speed of light from the headlights of a car traveling at 30mph is C and not C+30 because it is impossible to travel faster than light

It's the other way around. It's impossible to travel faster than light because the speed of the light from the headlights doesn't depend on the speed of the car.

In other words, you start with the postulate that the speed of light is independent of the speed of the source. If you're an observer standing beside the road, and the car is moving along that road, you will always measure the same value ##c## for the speed of the light coming from the headlights. This is equivalent to the car being at rest and you moving towards or away from it. Therefore no matter how fast you move, you will always measure the same value ##c## for the speed of light.

From there you can deduce that speed ##c## must also be your maximum possible speed. It goes something like this. Imagine chasing after a beam of light. No matter how fast your pursuit the beam will always recede from you at speed ##c##. Therefore you can never catch it.

stoomart

## 1. What is the speed of light?

The speed of light is a constant in physics, with a value of approximately 299,792,458 meters per second in a vacuum. This value is denoted by the symbol "c" and is an important concept in understanding the behavior of light.

## 2. How does light travel from car headlights and brake lights?

Light travels in the form of electromagnetic waves, which are created when electrons in atoms are excited and release energy. When a car turns on its headlights or brake lights, the energy from the car's battery is used to create these electromagnetic waves, which then travel outwards from the car at the speed of light.

## 3. Does the speed of light change when it travels from car headlights or brake lights?

No, the speed of light remains constant regardless of its source. This means that the light from a car's headlights and brake lights will also travel at the speed of light, regardless of the speed of the car or any other factors.

## 4. Can the speed of light be measured from car headlights or brake lights?

Yes, the speed of light can be measured using specialized equipment that can accurately measure the time it takes for light to travel a certain distance. However, due to the high speed of light, these measurements require very precise instruments and techniques.

## 5. How does the speed of light from car headlights or brake lights affect our perception?

The speed of light is so fast that we perceive it as instantaneous. This means that when we see a car's headlights or brake lights turn on, we don't notice any delay between the action and the light reaching our eyes. However, this does not mean that light travels infinitely fast, as it still takes time to travel from the source to our eyes.

Replies
20
Views
1K
Replies
2
Views
825
Replies
53
Views
4K
Replies
1
Views
1K
Replies
25
Views
3K
Replies
130
Views
9K
Replies
45
Views
4K
Replies
33
Views
3K
Replies
17
Views
941
Replies
22
Views
1K