Embedded Files
Chapter 16: Lesson 2 - Sources of Light with Math
Chapter 16: Lesson 2 - Sources of Light with Math
Equations:
Equations:
An important piece of paper to have handy when studying physics is your equation sheet. You never have to memorize any of the equations in class. You just have to know how to use them, manulipate them to solve for the unknown, and what each abbreviation stands for. A copy of the equation sheet is under resource materials on this website and on google classroom. It can also be found HERE. It is helpful to have a copy handy so you can mark equations on the sheet when we talk about them.
Speed of Light
Speed of Light
The speed of light is 3.0x108 m/s in a vacuum. It takes light less than one second to cross the entire earth. The speed of light is abbreviated with a c. Since c is a speed, we can substitute c for v in the equation, v=λƒ.
Luminous Flux
Luminous Flux
Luminous flux is the rate at which light is emitted from a source. We abbreviate it with a P. The unit for luminous flux is a lumen, abbreviated lm.
If you put a light bulb in a sphere, luminous flux is the rate at which light hits the sphere. A typical 100 W light bulb emits 1750 lm. A bulb can emit light is almost all directions. If you placed a light bulb in a sphere, the 1750 lm refers to all the light that would hit the sphere.
Illuminance
Illuminance
Illuminance is the amount of light that strikes a surface. It is abbreviated with E and the unit is lumens per square meter, (lm/m2) or lux, which is abbreviated lx.
Illuminance follows the inverse square law. A number of physical properties (like illuminance) get smaller as they get farther apart in a way that can be represented by an inverse square law. This means that the intensity of the light decreases as the distance between interacting objects increases.
The inverse square law is an important physics topic. For illuminance, it means that the light that strikes a surface 1 m away, is the same amount of light that will strike the surface 2 m away, but that light will strike a surface that is four times as large. The brightness of that light would be 1/4th as bright because it is spread out over a larger distance.
Other topics in physics use the inverse square law, such as the strength of the earth's gravitational field the intensity of light, and the strength of an electric field.
This video has a great explanation of the inverse square law. It is a quick explanation that is very important.
The illuminance, E, directly under a small light source, is given by this equation, where P represents the luminous flux of the source and d is the distance from the surface.
In the figure, the lightbulb is twice as far away from the screen as the candle. For the lightbulb to provide the same illuminance on the lightbulb side of the screen as the candle does on the candle side of the screen, the lightbulb would have to be four times brighter than the candle, and, therefore, the luminous intensity of the lightbulb would have to be four times the luminous intensity of the candle.
This is because distance is in the denominator and is squared in the equation above. Twice the distance is 4d and since 4 is in the denominator, the illuminance is 1/4 as much if the distance is doubled.
The difference between Luminous Flux and Illuminance
The difference between Luminous Flux and Illuminance
Luminous flux, which is measured in lumens, does not change with distance. It is how much light is emitted from the source per unit of time.
Illuminance, which is measured in lux, decreases the further away you are from the source.
Luminous Intensity
Luminous Intensity
Luminous intensity is the measure of light emitted by a source. It is abbreviated with a capital I and the unit is candela. Candela is a measure of luminance intensity, and is also referred to as candle power. Candela is abbreviate cd.
One candela is the light emitted from one candle, five candela is the light emitted by five candles.
Luminous intensity is equal to luminous flux divided by 4π.
Practice the Math!
Practice the Math!
Click here for the paper that I use in the video. As you watch this video, pause it and try the math on your own. When you have your answer, start the video and see if you are right. It is important to know where YOU make your mistakes so you can correct them for next time!
Remember there are more practice math problems under Chapter 16 Resources. Practice the math and see if your answer matches my answer.
Before the Next Class Period:
Before the Next Class Period:
Practice these Math Problems Before Class!
Practice these Math Problems Before Class!
Click on the picture to take you to the practice problems that I will be going over the next time we have class. Remember, you need to try the problems on your own to see where you make your mistakes if you want to learn how to do the math problems on your own.
You have finished Lesson 2!
You have finished Lesson 2!
Be sure to head over to google classroom and fill out the exit pass.
Report abuse