Lesson 7 calculating magnification & dichotomous keys

1. Calculating Magnification &
Dichotomous Keys

2. Learning Objectives
• Calculate the magnification of a drawing.
• Use a dichotomous key.
• Create a dichotomous key.

3. Calculating magnification.
Formula:
Magnification = size of the drawing
size of the real object

4. Calculate magnification video
• Magnification tips

5. Important things to remember!
• You must use the same units for all the
measurements.
– Usually, millimetres are the best units to use.
• You should not include any units for the final
answer. Magnification doesn’t have a unit but
you must include a times sign.

6. Have a go:
• Page 14 – Activity 1.2
– Questions: A1 & A2

7. Worksheet 2.3 – Magnification
Calculations
• Remember:
– Use the correct units of measurement
• Is it metres, millimetres or micrometres.

8. Worksheet 2.3 - Answers
• 1 magnification = image size ÷ actual size
= 36 ÷ 9 = × 4
• 2 actual size = image size ÷ magnification
= 80 ÷ 4 = 20 mm
• 3 image size = magnification × actual size
= 5 × 62 = 310 mm
• 4 45 mm = 45 × 1000 μm = 45 000 μm
magnification = image size ÷ actual size
= 45 000 ÷ 100 = × 450
• 5 15 mm = 15 × 1000 μm = 15 000 μm
actual size = image size ÷ magnification
= 15 000 ÷ 1500 = 10 μm
• 6 72 mm = 72 × 1000 μm = 72 000 μm
magnification = image size ÷ actual size
= 72 000 ÷ 0.9 = × 80 000

9. Keys and Identification
• Keys are used to identify different species. A
key will usually ask questions based on easily
identifiable features of an organism.
• Dichotomous keys use questions to which
there are only two answers. They can be
presented as a table of questions, or as a
branching tree of questions.

10. Get out your Homework:
Dichotomous Keys
Cambridge Biology Workbook:
• Page 5 & 6
• Using Keys
• Complete all questions.

11. Exercise 1.2 Using keys
Note that students cannot write in italic, so should
underline the binomials instead.
a
• A (given) 1b, 2a, 3a, Crocodylus niloticus
• B 1a, Geochelone elephantopus
• C 1b, 2b, Ophiophagus hannah
• D 1b, 2a, 3b, Chamaeleo gracilis
b
• i binomial
• ii The first part is the genus the organism belongs
to, and the second part is its species.
c
• They all have scales.

12. Writing a key

13. Worksheet 1.3 – Writing a key
• First of all:
– Look at the leaves on the worksheet.
– What features can you see?
• Leaf shape – lobes or not?
• Leaf edge – jagged or smooth?
• One main vein in the middle or more than one main
vein?
• Do the side veins come off in pairs or one at a time?
• Is the stalk longer than 1 cm?
• Anything else?

14. • SAPS GROUPING AND CLASSIFICATION

15. Homework
• Revise Unit 1
• On Monday we will review Unit 1.
• Write down any questions you have about the
work covered so far and we will discuss them
on Monday.