calculating the actual size of an object · 2021. 3. 22. · a calculating the actual size of an...

Skill building pack: Calculation Calculating the actual size of an object (For teachers)

New Senior Secondary Mastering Biology 1 © Oxford University Press

(Third Edition)

Skill building pack: Calculation

Calculating the actual size of an object

Microscopes are important tools for scientists. They produce a magnified image of an object.

Magnification is often indicated on a photomicrograph. We can find out the actual size of the object

shown using the magnification given.

A Calculating the actual size of an object using magnification

Magnification represents how many times an image is larger than the object. To calculate the actual

size of an object shown in a photomicrograph, we can use the equation below:

Below is an example showing how to calculate the actual size of an ovum shown in a photomicrograph.

Steps Workings

1 Measure the diameter of the ovum (XY)

in the photomicrograph using a ruler.

Diameter of the ovum (XY) in the photomicrograph

= 4.1 cm

2 Find out the magnification of the

photomicrograph.

3 Substitute these values into the equation.

Magnification = 400

Magnification =

400 =

4 Give the answer using a suitable unit. Size of the object = 0.01025 cm

= 0.10 mm

The actual diameter of the ovum is 0.10 mm.

size of the image

size of the object

4.1 cm

size of the object

Magnification = size of the image

size of the object

(×400)

X

Y



(Third Edition)

Practice 1

The photomicrograph below shows a red blood cell in a capillary. Calculate the actual diameter of

the red blood cell (XY).

Diameter of the red blood cell (XY) in the photomicrograph = 4 cm

Magnification = 8000

Magnification =

8000 =

Size of the object =

Size of the object = 0.0005 cm = 5 μm

The actual diameter of the red blood cell is 5 μm .

(×8000)

size of the image

size of the object

4 cm

size of the object

4 cm

8000

X

Y



(Third Edition)

Practice 2

The photomicrograph below shows a bacterium that causes cholera in humans. Calculate the actual

length of the bacterium (XY).

Length of the bacterium (XY) in the photomicrograph = 9.4 cm

Magnification = 15000

Magnification =

15000 =

Size of the object =

Size of the object = 0.000627 cm = 6.27 μm

The actual length of the bacterium is 6.27 μm.

size of the image

size of the object

9.4 cm

size of the object

9.4 cm

15000

(×15 000)

X

Y



(Third Edition)

B Calculating the actual size of an object from a scale bar

Sometimes, a scale bar instead of magnification is given on a photomicrograph. In this case, we

have to calculate the ratio of the length of the object to the length of the scale bar, in order to find

out the actual size of the object.

Below is an example showing how to calculate the actual length of a pollen tube shown in a

photomicrograph using a scale bar.

Steps Workings

1 Measure the length of the pollen tube (PQ)

in the photomicrograph using a ruler.

Length of the pollen tube (PQ) in the

photomicrograph

= 4 cm

2 Measure the length of the scale bar using a

ruler.

Length of the scale bar

= 1 cm

3 Divide the measured length of the pollen

tube by the length of the scale bar to find

out their ratio.

Ratio of the measured length of the pollen tube to

the length of the scale bar

= 4 cm / 1 cm

= 4

4 Multiply the answer in step 3 by the actual

length that the scale bar represents.

Actual length of the pollen tube

= 50 μm × 4

= 200 μm

50 μm

P

Q



(Third Edition)

Practice 3

The electron micrograph below shows a yeast cell commonly used in baking. Calculate the actual

length of the yeast cell (PQ).

Length of the yeast cell (PQ) in the photomicrograph = 7.6 cm

Length of the scale bar = 1 cm

Ratio of the measured length of the yeast cell to the length of the scale bar

= 7.6 cm / 1 cm

= 7.6

Actual length of the yeast cell

= 2 μm × 7.6

= 15.2 μm

2 μm

P

Q



(Third Edition)

Practice 4

The electron micrograph below shows a chloroplast. Calculate the actual length of the chloroplast

(PQ).

Length of the chloroplast (PQ) in the photomicrograph = 7 cm

Length of the scale bar = 1 cm

Ratio of the measured length of the chloroplast to the length of the scale bar

= 7 cm / 1 cm

= 7

Actual length of the chloroplast

= 600 nm × 7

= 4200 nm

= 4.2 μm

600 nm

P

Q

技巧提升教材：運算計算物體的實際大小

（教師用）

新高中基礎生物學（第三版） 1 © 牛津大學出版社

技巧提升教材：運算

計算物體的實際大小

顯微鏡是科學家重要的工具，能產生物體的放大影像。顯微照片通常註明放大率，我們可以

根據這項資料來計算照片所顯示的物體的實際大小。

A 利用放大率計算物體的實際大小

放大率表示影像比原來的物體放大了多少倍。計算照片所顯示的物體的實際大小時，可利用

以下的方程式：

下面是一個例子，示範怎樣計算顯微照片所示的卵的實際大小。

步驟計算步驟

1 用間尺量度顯微照片上卵的直徑 (XY)。顯微照片上卵的直徑 (XY) = 4.1 cm

2 找出顯微照片的放大率。

3 把以上數值代入方程式。

放大率 = 400

放大率 =

400 =

4 用合適單位表示答案。物體大小 = 0.01025 cm

= 0.10 mm

卵的實際直徑是 0.10 mm。

影像大小

物體大小

4.1 cm

物體大小

放大率 = 影像大小

物體大小

(×400)

X

Y


（教師用）


練習 1

以下顯微照片顯示微血管內一個紅血細胞。計算這個紅血細胞的實際直徑 (XY)。

顯微照片上紅血細胞的直徑 (XY) = 4 cm

放大率 = 8000

放大率 =

8000 =

物體大小 =

物體大小 = 0.0005 cm = 5 μm

紅血細胞的實際直徑是 5 μm 。

(×8000)

影像大小

物體大小

4 cm

物體大小

4 cm

8000

X

Y


（教師用）


練習 2

以下顯微照片顯示一個能引致霍亂的細菌。計算這個細菌的實際長度 (XY)。

顯微照片上細菌的長度 (XY) = 9.4 cm

放大率 = 15000

放大率 =

15000 =

物體大小 =

物體大小 = 0.000627 cm = 6.27 μm

細菌的實際長度是 6.27 μm。

影像大小

物體大小

9.4 cm

物體大小

9.4 cm

15000

(×15 000)

X

Y


（教師用）


B 利用比例尺計算物體的實際大小

有時候，顯微照片上註明的是比例尺而非放大率。在這個情況下，我們便需要計算物體長度

和比例尺長度的比例，從而找出物體的實際大小。

下面是一個例子，示範怎樣利用比例尺來計算顯微照片所示的花粉管的實際大小。

步驟計算步驟

1 用間尺量度顯微照片上花粉管的

長度 (PQ)。

顯微照片上花粉管的長度 (PQ) = 4 cm

2 用間尺量度比例尺的長度。比例尺的長度 = 1 cm

3 把花粉管的量度長度除以比例尺

的長度來找出兩者的比例。

花粉管的量度長度和比例尺長度的比例

= 4 cm / 1 cm

= 4

4 把步驟3的答案乘以比例尺所代表

的實際長度。

花粉管的實際長度

= 50 μm × 4

= 200 μm

50 μm

P

Q


（教師用）


練習 3

以下電子顯微照片顯示一個常用於烘焙的酵母細胞。計算這個酵母細胞的實際長度 (PQ)。

顯微照片上酵母細胞的長度 (PQ) = 7.6 cm

比例尺的長度 = 1 cm

酵母細胞的量度長度和比例尺長度的比例

= 7.6 cm / 1 cm

= 7.6

酵母細胞的實際長度

= 2 μm × 7.6

= 15.2 μm

2 μm

P

Q


（教師用）


練習 4

以下電子顯微照片顯示一個葉綠體。計算這個葉綠體的實際長度 (PQ)。

顯微照片上葉綠體的長度 (PQ) = 7 cm

比例尺的長度 = 1 cm

葉綠體的量度長度和比例尺長度的比例

= 7 cm / 1 cm

= 7

葉綠體的實際長度

= 600 nm × 7

= 4200 nm

= 4.2 μm

600 nm

P

Q

calculating the actual size of an object · 2021. 3. 22. · a calculating the actual size of an...

Documents