The Speed of Light How fast does light
- Slides: 8
The Speed of Light
How fast does light travel? • The transmission of light is not instantaneous! • Light takes time to travel from a light bulb to your eye. • Despite light being the fastest thing known, the speed of light can still be measured! • The speed that light travels is 300, 000 km/sec or 186, 000 miles/sec!
What is a light year? ? ? • The distance light travels in one year is called a light year! • This is an astronomical unit astronomers use to express how far away an object is!
So…how far does light travel in a year? ? • 300, 000 km/sec x 60 sec/min x 60 min/hr x 24 hr/day x 365 days/yr = 9, 460, 800, 000 km or 9. 5 trillion kilometers • 186, 000 mi/sec x 60 sec/min x 60 min/hr x 24 hr/day x 365 days/yr= 5, 865, 696, 000 mi or 6 trillion miles
Can we see back in time? • Yes! • Remember light travel is not instantaneous, therefore when a photon of light leaves a star it takes time to travel to your eye. • The time it takes depends on its distance from Earth. • It takes less time for you to travel to Sacramento than to LA, therefore a star that is farther away will cause the photon of light to take a longer time to reach your eye.
Looking back into time • How far away an object is determines how long ago we are seeing back in time. • Example: The North Star (Polaris) is 360 light years away. Take the year that it is now and subtract the distance from the year 2010 -360 l. y. = the year 1650 A. D. If your answer is a negative number, then it occurred in B. C. • In other words you are seeing Polaris as it was back in the year 1650 A. D.
Looking into the Future! • So when will you see the photon of light that is leaving the star at this very moment? • Example: Polaris is 360 l. y. away. Take the year that is us now and add the distance to the year. – 2010 +360 l. y. = 2370 A. D. • So in the year 2370, you will finally see the photon of light that left Polaris today! Wow! • Challenge: What is the distance in miles to Polaris? Kilometers?
Answer to the Challenge • Miles: – 360 l. y. x 6 trillion miles = 2. 16 x 1015 • Kilometers: – 360 l. y. x 9. 5 trillion km = 3. 4 x 1015
