1. Lesson 21 - Optical Devices
The earliest optical devices
Everyday optical devices
Telescopes… Refractive and Reflective
Microscopes/Glasses/Binoculars

2. Review of Optical Devices
An Optical device is any technology that uses light
Optical devices can be as simple as a mirror or as complex as the Hubble telescope
The invention of optical devices has lead to many great discoveries that have shapes our daily lives

3. Purposes of Optical Devices
The purpose of any optical device is to collect and gather light
Whether its for telescopes, cameras, microscopes, or simply our eyes… they are designed to gather light

4. Views from Hubble

5. Purposes of Optical Devices
Without Hubble’s ability to collect light from a huge area and focus into an image, we would never have enough light to see those amazing pictures
The greater the light gathering ability, the more detail we can see from further away objects

6. Comparing Gathering

7. How Much Better Is It?
Below we see two light gathering devices, each with a different diameter… How many times more, or how much more light can the larger opening gather?
d = 10 cm
d = 5 cm

8. Calculating Surface Area
Surface area of a Circle:
SA = 3.14 x r²
Example
SA = 3.14 x r² SA = 3.14 x r²
SA = 3.14 x 2.5² SA = 3.14 x 5²
SA = 3.14 x 6.25 SA = 3.14 x 25
SA = cm² SA = 78.5 cm²
78.5 ÷ = 4 Therefore the larger telescope with twice the diameter actually has 4 times the light gathering ability
d = 5 cm
d = 10 cm

9. Telescopes There are two types of telescopes: REFLECTIVE REFRACTIVE
Refractive telescopes use lenses to bend the light to a focal point for our eyes
Reflective telescopes use a concave mirror to reflect the light to a focal point for our eyes
Which is better… WHY?

10. Telescopes
There are two types of telescopes:
REFLECTIVE REFRACTIVE

11. PROS & CONS
Refracting telescopes are cheaper to make… but there are draw backs
Both telescopes can be made to gather equal amounts of light… However, refractive telescopes suffer from CHROMATIC ABERRATION so after a certain point they blur the image to much
Reflective telescopes don’t have this problem so they can be made as large as possible and the image will still be intact

12. Chromatic Aberration
Chromatic Aberration is the tendency for different coloured light to bend at different amounts during refraction
Therefore, when you pass you coloured light through a lens, all the light is not bent to the same spot and the image is blurred

13. Chromatic Aberration

14. Microscopes
Microscopes are also designed to gather light, but they do so from a very small area up close
Microscopes collect what little light there is at the microscopic level and then expand the image using refraction for us to see
We can only see tiny objects to a certain point before their just isn’t enough light for our eyes to detect an image

15. Microscopes
Most microscopes use a light source at the base to shine onto the specimen from below because without it there would not be enough light for our eye to see
You will need to do more research on microscopes in the next unit

16. Other Optical Devices
Magnify glasses help enlarge light by gathering light from a small area and enlarging it
Eye glasses refract gathered light to focus it on to our retina when our own lens cannot
Convex mirrors allow us to see a large surface area on a small space
Concave mirrors allow us to reflect light to a focal point

17. Homework Tonight
You are being given a worksheet that covers the material we just went over
Use the sheets as study tools and for practice questions that could be on the Unit End Exam
We will be going over this worksheet tomorrow!!

18. Textbook Pages
You may look at the following textbook pages to help you with your assignment… they compliment what we have talked about today
Use pages in the Science Focus 8 Textbook to help you complete the worksheet.
Use pages in the Science in Action 8 Textbook to help you complete the worksheet.