PHYS 1407 – Conceptual Physics II

Refraction

 

Leader: _________________________          Recorder: __________________________

Skeptic: _________________________         Encourager: ________________________

 

Materials

Optics Bench with table, protractor and light source

Pasco Ray Optics Kit

Semicircular Cylindrical lens

Paper

Ruler

 

Introduction

      In this activity we will examine what happens to light when it hits a boundary between two surfaces at an oblique angle.  The phenomena is called refraction.

 

Part 1  Refraction

Procedure

1.  Set-up

Adjust the mask on the light source so that a single ray comes out which hits the center of the protractor on the optics table.  Adjust the protractor on the optics table so that word Normal faces directly towards the light source. Place the semicircular cylindrical lens with its flat side facing the light source and so it is aligned along the axis of the protractor.

 

2.  Preliminary observations.

Align the ray box so that the light hits the center of the protractor.  This gives you an incident angle of 0°.

 

Q1)  Describe the path of the light in the lens.

 

 

Now rotate the table so that the light hits with an incident angle of 20°.

 

Q2)  Describe the path of the light in the lens now.

 

 

Q3)  What happens to the direction the light travels at the flat surface of the lens?

 

 

Q4)  Does the light follow a straight line or a curved path in the lens.

 

 

Q5)  Draw a sketch of the light as it travels from the air into the lens.

 

 

3.  Data acquisition

Align the ray box so that the light hits the center of the protractor.  Starting with an incident angle of 0°, determine where the beam exits the lens.  Record the incident and refracted angles in the data table below.  Be sure to measure both angles from the normal.  Determine the sine of the incident and refracted angles and record that value in the data table as well.

 

4.  Repeat the Procedure

Rotate the table by 10° and repeat the procedure.  Make sure that the incident ray hits the center of the protractor each time.  You may have to apply a push slightly on the light source to obtain this.  Record the incident and refracted angles in the data table.  Repeat the procedure increasing the incident angle by 10° each time until reaching 80°.  Record your data in the table below.  Make sure that the incident and refracted angles are measured from the normal.

 

Data Table

Incident Angle, qi

Refracted angle, qr

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Q6)  In the table above, are the incident or refracted angles larger?

 

 

Q7)  Does the light make a bigger angle with the surface in the air or in the lens?

 

 

5.  Reverse the orientation of the lens

Keep the lens aligned along the axis of the protractor but rotate the table a half turn so that the semicircular side faces the light source. Make sure that the incident ray hits the center of the protractor each time.  You may have to apply a push slightly on the light source to obtain this.  Record the incident and refracted angles in the data table.  Repeat the procedure increasing the incident angle by 10° each time until reaching 60°.  If you note anything that strikes you as odd, you might refer to Q10) below.  Record your data in the table on the next page.  Make sure that the incident and refracted angles are measured from the normal.

 


Data Table

Incident Angle, qi

Refracted angle, qr

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Q8)  In this case the light came from the glass and went into the air.  Which was bigger, the angle in the lens or in the air.

 

 

Q9)  Compare your answer for Q8) to Q7).  What do these answers suggest?

 

 

Q10)  Did anything odd happen at an incident angle around 40° to 50°?  Describe it.

 

 

When light travels through matter, it travels at a different speed than it does in a vacuum.  The speed light travels through a material is characterized by a property of the material called the index of refraction.  The speed light travels in a material is given by

speed = 300,000,000 m/s ÷ index of refraction. 

 

Q11)  The index of refraction for the Plexiglas from which the lens is made is about 1.4.  How fast does light travel in the Plexiglas?

 

 

Q12)  The index of refraction of air is about 1.0.  How fast does light travel in air?

 

 

You should have observed that angle in the air was bigger than the angle in the lens.  Thus it is possible when light is incident from the lens into the air for the angle in the air to approach 90°  At that point instead of refracting the light will reflect.  This process is called total internal reflection.  The angle at which the light first starts to totally internally reflect is called the critical angle.

 

Q13)  Measure the critical angle by rotating the protractor and finding the incident angle at which the light first starts to totally internally reflect.  Record the value you found.

 

 

 

 

Part 2  Refraction by Lenses

Procedure and Questions

Adjust the mask in front of the light source so that you obtain four parallel rays.  Remove the semicircular cylindrical lens and place the white piece of paper on top of the optical table.  Place the biconvex lens in front of the rays on top of the white sheet of paper.  Trace the incident and refracted rays on the paper using a straight edge.

 

Q14)  Sketch how the rays look.

 

 

 

Q15)  Do the rays seem to come to a focus?  If so determine the focal length.

 

 

Reverse the orientation of the lens.

 

Q16)  Do the rays behave the same regardless of orientation of the lens?

 

 

Q17)  Measure the focal length of the lens in this orientation.  Is it the same as before?

 

 

Replace the biconvex lens with the biconcave lens.

Q18)  How do the rays passing through the biconcave lens compare to those passing through the biconvex lens?

 

Q19)  Fill in the following blanks with either diverging or converging.  A concave lens produced _________________ rays, and thus a concave lens is also known as a ____________________ lens.  A convex lens produced _________________ rays, and thus a convex lens is also known as a ____________________ lens.

 

For the biconcave lens, use a straight edge to trace the incident and refracted rays on the white piece of paper.  Backtrack the rays passing through the biconcave lens.

 

Q20)  Do the backtracked rays appear to come together at a point?

 

 

Q21)  Determine the focal length of the biconcave lens.

 

Reverse the orientation of the lens.

 

Q22)  Do the rays behave the same regardless of orientation of the lens?

 

Q23)  Measure the focal length of the lens in this orientation.  Is it the same as before?

 

You should have noticed that a lens has the same focusing properties if it is reversed.  We describe this property of lenses by saying that they have two focal points - one in front of the lens and one behind.

 

Q24)  Sketch a diagram of a converging lens and indicate its front and back focal points.

 

 

 

Q25)  Sketch a diagram of a diverging lens and indicate its front and back focal points.