Light:reflection

We see luminous objects because the light they give out comes to our eyes. Everything else we see because of reflection. Light reflects in very different ways from a smooth object compared to a rough one.

Scattering

Rought objects scatter light. The light comes in parallel but bounces off in all directions. This is why we don't see an image in them.

The law of reflection

Smooth surfaces scatter light according to the law of reflection. To do this the surface must be really smooth, like a mirror, a still lake or a plane of glass.

When this happens the light reflects as in this diagram. The angle of incidence = the angle of reflection when we measure it from the normal (which is an imaginary line at 90 degrees to the reflecting surface).

This means light rays that come in parallel leave the surface parallel and you get an image.

The window reflects some of the light from the flowers so you get a dim image compared to a mirror. But why are the flowers on the right here reflected 3 times and those on the left only once?

Light : Basics

Light is a form of energy. What we call light is a small part of the electromagnetic spectrum which includes ultra violet and infrared as well as radio waves and microwaves. All these things are basically the same but we can only see a small section of the whole lot (spectrum). This make sense, when we were evolving the only radio waves on the planet would have come from the stars or lightning. Not helpful to see only those things when a predator is creeping up on you. The difference between all these parts of the electromagnetic spectrum is their wave length and our eyes evolved to see the most useful wavelengths around on this planet.

Remember light travels in straight lines (unless it goes past a star or black hole when they can be bent). In space light travels at 300,000,000 m/s around a million times faster than sound travels through the air. Luminous objects are light sources they give out light. Materials can be described as transparent (let's through all the light), translucent (let's through some light) or opaque (let's through no light). Remember the thickness of the material is important! One sheet of paper is translucent but 100 sheets will be opaque. We also need to know about reflection, refraction and dispersion which are coming in the next post.

Speed

Speed measures how far we travel in a unit of time.  This is why we measure it in miles per hour.  Miles measure distance per means in every and hours measure time.  Of course there are other units of speed as well such as kilometres per hour and meters per second which we use a great deal in common entrance physics. Fast and slow moving things might need different units. A rocket will move a certain number of kilometres per second (around 11 km/s to escape earth orbit) a glacier might move a few
centimetres/year. It would be silly to measure these movements in meters per second the numbers would be really big or really small and any calculations you do complicated.
So to measure speed we need to know the distance moved and the time taken. Then we divide the distance by the time. In other words:
Speed= distance / time
If you forget this just think of a speed you go in your car. 50 miles per hour. Miles (distance) per (divided by) hours (time).

I am on a train as I write this we are going 60 miles in 2 hours so the speed is
S=d/t
S=60/2=30 mph

Remember this will be our average speed. We stop at each station, speed up and slow down all the way so we won't go at 30mph exactly much of the time.

If my train was travelling for 4 hours at this average speed how far would it go?
Well the distance=speed x time
Distance =30 x 4 =120 miles.
This is because the speed tells us the distance we go each hour if I go 4 lots of 30 that is 120 miles.
So how long would it take my train to do 105 miles?
Here we do time=distance/speed
Time=105/30=3.5 hours.
Take care. Exam question setters love to put in traps. Look at this question.
A horse travels at 8m/s for two minutes how far does it go?
Here I have muddled up minutes and seconds. The first thing we must work out is how many seconds the horse is travelling for. 2x60=120 (as there are 60 seconds in one minute)
Now we can do
distance=speed x time
Distance=8 x120
Distance=960m

Series and parallel circuits

See our videos

Series and parallel circuits have several key differences. Firstly series circuits have only one current path around them. There are no branches. Parallel circuits have two or more current paths. There are different routes around the circuit. See our videos on YouTube to get the idea. Series circuits must have everything turned on at once but parallel circuits can have different things working at different times, this means we can use them to make And and Or circuits as well as many others. In a series circuit the current will be the same wherever you measure it. In a parallel circuit the current will split down different branches. Adding together the current in the different branches will tell you the current leaving the cells or returning to it.